The Torah commands us to be kedoshim (holy, or set apart), requiring purity in what we eat, how we speak, what we wear, and how we behave. Tum'a is spiritual impurity generally resulting from transitions from life to death (even in a small measure; for instance—sleep or cutting nails). Tum'a inhibits or blocks us from achieving holiness.
Since we do not have a “red heifer” with which to make purifying water solutions, all people are considered to have some level of tum'a today.
Although there are three reasons to ritually wash hands--to add kedusha; to remove tum'a; and to remove dirt--tum'a normally has nothing to do with physical dirt. There are many types and levels of tum'a, with no exact progression. The following guideline is approximately in descending order from most impure to least:

Sources of the Different Levels of Tum'a

• Dead body (touching or being in same building with a dead body);
• Cemetery;
• Carcass of any dead animal not slaughtered by shechita;
• Women during and after menstruation or after childbirth (but before they immerse in a mikva);
• Sleep;
• Possibly a bathroom;
• Your hands' transferring tum'a to wet food;
• Your hands' transferring tum'a to bread;
• Food from under a bed on which someone slept;
• Intercourse or seminal emission;
• Having your beard, hair, or nails cut;
• Leather shoes;
• Touching body parts.

Depending on the level of tum'a, purifying may require:

• Washing your hands by the Three-Times Method.
• Washing your hands by the One-Time Method.
• Immersion in a mikva. OR
• Sprinkling with water that had been treated with ashes from a red heifer (which we do not have now).

By Sara-Malka (Diane) Laderman

Kosher (Hebrew for “fitting” or “suitable”) means foods that comply with certain laws. Kosher rules could be summed up like this:

• The food must start out kosher. 
• The food must stay kosher during processing.

Starting Out Kosher
The Food's Natural State

Rule #1

Plants
All Plants, Raw, Are Inherently Kosher

All raw, unprocessed plants are kosher. However, restrictions on produce grown in Eretz Yisrael may apply (teruma, ma'aser, shmita), and orla may apply to produce grown anywhere in the world.

• For laws about eating perennial fruits, see appropriate listings under Agriculture. 
• For laws regarding bugs in plant produce, see below.

Rule #2

Mammals
All Mammals that Chew Their Cud and
Have Split Hooves Are Inherently Kosher

Kosher mammals are all cud-chewing, split-hooved animals (Leviticus/Vayikra 11:1-8 and Deuteronomy/Devarim 14:3-8). Included are both domestic ("beheimot"--goat, sheep, and cow families ) and wild ("chayot"--deer, giraffe, and wild goat and sheep families) mammals. There are two (sometimes) practical differences between the two groups:

• You may eat the cheilev (a type of fat) from a wild kosher mammal, and
• After slaughtering, you must cover the blood from a wild kosher mammal but not a domesticated kosher mammal.

Below is a sampling of kosher mammals:

Hooves

Q: How can you tell if an animal has split hooves?

A:
1)  Split Hooves Must Be Hooves

Hooves must be made of hoof material--a hard substance similar to your fingernails—not fleshy feet.

2)  Split Hooves Must Be Split

Hooves must be split all the way through from front to back.

Cud-Chewing

Q:  How can you tell if an animal chews its cud?

A:  Watch for the sliding ball.

When a cud-chewing animal starts to eat, you will see it bolting down its food into its first stomach, like a hungry 9th grade boy (much like humans racing to throw groceries into their shopping carts), in case a lion or bear is coming to eat him or her.

Next, it will find a safe place to more leisurely bring up its cud and chew its stash. During cud-chewing time, especially for goats (sheep are usually too woolly to make out shapes), you will distinctly see:

• Racketball shape popping up the goat's throat, 
• Goat's cheeks ballooning out and its lower jaw chewing in a horizontal figure-eight pattern, and, a little later,
• Racketball shape sliding down the throat again.  

You will soon see the shape of a new racketball pop up the throat.

By contrast, a non-kosher animal will chew slowly and well the first time—it will not have another chance to chew its food later, like the kosher animals do.  

Imposters

Animals in the camel family (camel, llama, alpaca, vicunya, etc.) appear to have split hooves when seen from the front.  These are actually just two long toenails in front of a padded, fleshy, incompletely split foot, which you can easily distinguish as a whole foot when looking from the back.

One non-kosher animal has great-looking split hooves but doesn't chew its cud—animals from the pig family.

Insight from Masechet Chullin

All kosher mammals inherently have horns; all non-kosher animals are hornless.  Bottom line:  If you find a horned animal, it's definitely kosher.

But horns are not a halachic requirement from the Torah like split hooves and cud chewing are, which is a good thing, since some breeds of goats, sheep, and cows are naturally “polled” (born hornless) or their horn buds were removed when they were young to prevent damage later.

Rule #3

Fowl
All Fowl That Have “Masoret” Are Inherently Kosher

Not everyone's agreed as to what the Torah means by a “netz” or a “yanshuf.”  So when Leviticus/VaYikra 11:13-19 lists the 20 non-kosher flying species—allowing us to eat anything NOT on the list—we ignore the list and just eat what we know our ancestors traditionally ate as kosher.  This tradition is known as masoret.

In the US, we eat all breeds of chickens and--in most circles--turkey, all breeds of goose except those whose beak is black (such as the Canadian goose) or whose beak does not go straight back to its forehead (like the Chinese goose), and Peking duck (we don't eat mallard or Muscovy ducks or their close relatives).

In Israel, additional birds eaten as kosher include mallard and Muscovy ducks, guinea fowl, Couternix quail, pigeons, and turtle doves.

Note Some Jewish families originating in Germany, Iran, and other places maintain their masoret on eating pheasant, and you may be able to receive masoret on various species from researchers such as “The Aris”--Dr. Ari Greenspan and Rabbi Dr. Ari Zivotofsky, both Jewish ritual slaughterers (shochtim) who have spent the last 20 years interviewing and videotaping elderly European and Sefardi immigrants to Israel as to what birds they ate as kosher in their home countries. You can google their work or read some of Dr. Zivotofsky's articles on www.kashrut.com.

Zivchei Cohen, a book written and published by a Jewish ritual slaughterer (shochet) in Italy, shows colored illustrations of 29 species known to be kosher, including peacock, pheasant, Couternix quail, mallard duck, and numerous songbirds.  Maor L'Masechet Chullin U'Vechorot (vol. 2, Feldheim, pp. 29-33) reproduces these colorful illustrations and names each bird in five languages, noting that the 29 were listed to acquaint students of Jewish ritual slaughter (shechita) only with rarer birds' identities and that the well-known kosher species were not included in the 29!

Chazal noted that kosher birds share certain characteristics:

• They sit on a branch with three toes in front and one in back.  Non-kosher birds usually sit two and two, as they need equal strength on both sides of their feet for killing and carrying off food, except for:
  • Owls, whose feet are flexible and can move their toes to the side, forward, or back, and 
  • Vultures, who need balance walking instead of gripping, since they walk on the ground to eat food that is already dead.
• They lay eggs that are not entirely round or oval but are, well, egg-shaped, with kad v'chad—a rounded end and a pointed end. Not all egg-shaped eggs are kosher, but all totally round eggs, if from fowl, are not kosher (fish eggs from kosher fish, which are perfectly round, are of course kosher). There are some eggs, including from doves, that seem perfectly oval but are actually kosher.

Rule #4

Fish
All Fish That Have Fins and Scales Are Inherently Kosher

This excludes most eels (some conger eels that have kosher scales are kosher!) and all shellfish, catfish, sharks, swordfish, sea urchins, jellyfish, sea slugs, and many other sea creatures.

In addition to commonly eaten kosher fish such as salmon and tuna, some unexpected fish are also kosher, including barracuda, goldfish, and many other pet and tropical fish.

Rule #5

Grasshoppers

All other creatures, except the four kosher locusts, are not kosher. Rule #6

Kosher from Kosher
Whatever Food Substances Come Out of a Kosher Animal Are Inherently Kosher…

except for some fats (cheilev), blood, and the sciatic nerve (gid ha'nashe).

Milk from a cow (a kosher animal) is kosher. Milk from a pig (a non-kosher animal) is not. An egg from a kosher bird is kosher; an egg from a non-kosher bird is not kosher.
Exception

Q:  Since bees are not kosher, how can we eat honey? 

A:  Honey is not produced from bee parts, but rather from flower parts.

Rule #7

Animal Blood
May Not Be Eaten in Any Form.

Note Fish blood is not forbidden.

Preparing Kosher
Harvest and Kitchen

Plants

What To Check

• Remove bugs (see Why Bugs May Not Be Eaten)
• Select fruits and vegetables that have no harvest-related problems such as orla (and in Eretz Yisrael, kilayim, shmita, etc.); separate out teruma and ma'aser from any Israeli-grown produce that requires it (see Teruma/Ma'aser: Ownership: What Is Hefkeir Produce)
• Make sure that any liquid grape product to be handled by a non-Jew for a Jew has been cooked or pasteurized before being handled.  Cooking turns the wine into an inferior product disqualified for use in idolatrous practices.

Animals

Mammals

Slaughter/Shechita

Kosher mammals must be slaughtered in the quickest and most humane manner possible, according to halacha.  A highly trained ritual slaughterer (shochet) must perform the slaughtering (“shechita”).  He checks the knife before the slaughtering to ensure there are no burrs to catch on the animal's throat.  He says the blessing “al ha'shchita” and then cuts the windpipe and the esophagus as well as the neck arteries.  After slaughtering, he checks the knife again for burrs (if he finds one, the animal is not kosher) and checks the animal's lungs to make sure the animal wasn't about to die of lung perforation in the near future.  

Certain types of adhesions may be found on the animal's lungs. If they can be removed (by peeling) without perforating the lungs, the meat is kosher. If there are only small and easily removed lesions, the meat is glatt (“smooth”). If there are no lesions at all, the meat is classified as “Beit Yosef.”

Kosher lamb and goat are always glatt/chalak kosher.

Actually, there are 18 organic or physical defects that may make meat non-kosher but, as a practical matter, we only check for lesions in the lungs and also in the second stomach. 

If the animal proves to have been healthy, it is sometimes hung upside down to allow the arterial blood to drain out. (It is possible to hang the animals before being slaughtered but this is not the usual method). 

Skinning and Traiboring

The animal is skinned.

Next, the animal is traibored. Traiboring removes certain nerves, sinews, blood vessels, and fats that we don't eat, including the sciatic nerve damaged when our forefather Jacob wrestled with the angel at the Jabbok stream.  

In the US, only the forequarters are traibored and eaten, and the hind portion is sold to the non-Jewish consumer. In Israel, the hind portion is traibored too and eaten as kosher.

May you traibor meat once it's cooked?  And if not, how did Jews traibor more than 1 million Passover lamb offerings that had to be slaughtered and prepared between midday and evening (and it takes 2-3 hours to traibor one lamb!). The Jewish commentator The Raavad says the Passover lamb was traibored before roasting; Rambam disagrees, since the lamb had to be roasted whole. Rambam opines that the sinew, unlike fat, does not impart its flavor to the meat and that people would just traibor the Passover offering meat on their plates.

Removing Blood

The next steps involve removing blood (“kashering”) and can be done at the butcher's or at your home.  The meat is cut, rinsed, soaked for at least 30 minutes, put on a slanted board to allow the blood to run off, and covered with kosher (a coarse) salt for one hour.  After being rinsed three more times, the meat is now kashered.

Preparing the Liver

The liver is cut halfway through several times and covered with kosher salt top and bottom.  You can oven broil the liver on a rack reserved for that purpose. The blood must be able to drain away from the liver 

You can instead broil the liver over a fire outdoors.  Grilling outside will give the liver a delicious smoky flavor that even children like--but do NOT allow the neighborhood cats to steal your livers off the grill!  

Fowl

Covering Blood

Kosher fowl is slaughtered and, when it stops flapping, is usually hung upside down to allow the arterial blood to run out and onto the earth. Cover all the blood with dirt (a mitzva from the Torah--mitzva d'oraita) and say the blessing “al kisuy dam b'afar.”

Defeathering

Rinse with water and remove the feathers. Defeathering can take a while for chickens and up to two hours for one small duck, especially if you are saving the down!  

Removing Internal Organs

Rinse the bird. Usually, a circle of flesh surrounding the anus is cut out.  Start pulling out the digestive system.  Recognizable items such as the liver, heart, and giblets will come out and eventually you will be able to stick in your hand and pull out the lungs.  This is not as cold and unpleasant as it sounds because the bird will be warm for quite a while.

Salting

Once the bird is defeathered and the internal organs have been removed, rinse and salt with kosher salt inside and out and put it on a slanting board for an hour. Rinse three more times and cook!

Preparing the Giblets

Cut off the hard coating at one end of the giblets and rinse out the fine sand within. Remove the yellow internal lining.  Salt and kasher with the rest of the bird.

Preparing the Liver

To kasher the liver, see Preparing the Liver, above, for meat liver.

Fish

Buying Fish

Kosher fish bought from a store in which non-kosher fish are also sold should have any cut surfaces scraped and should be rinsed before using. Ideally, the knife that cuts the fish should be washed with soap and water beforehand. 

Grasshoppers

Chagav Grasshoppers

Not much preparation needed here. Many Yemenites just twist off their heads and eat. B'tei'avon!

Substances from Animals

Milk

Dairy must be kept separate from meat, with a separate set of pots, pans, servers, scrubbers, and dishpans each for dairy and meat. See Kashrut: Dairy/Meat Combinations.

Eggs

Eggs must be checked for blood spots.  Throw out a fertilized egg with a blood spot. You may remove the blood in the white of the egg and eat the rest of an unfertilized egg, but the custom is to not eat the egg at all.

Unwanted Additives

Manufacturing Aids

In the US, food manufacturers are allowed to add “manufacturing aids”--even more than 1/60th of the volume of the other ingredients--without listing them. Some foods therefore need special supervision to ensure non-kosher substances have not been added.

Examples

• Kosher oils may be deodorized by heating them in vats that previously contained non-kosher oil, which renders the formerly kosher oil non-kosher. Or they may be put into tankers previously used for non-kosher liquids.
• Food colorings may come from the cochineal insect, which is non-kosher, and flavorings may be derived from the musk of non-kosher animals.
• Cheeses may have non-kosher rennet or pig milk added. Also, the rabbis of thousands of years ago made an injunction that even where the ingredients are kosher, cheese still requires kosher supervision.
• Maple syrup in the vat may be stirred with bacon (which is non-kosher) to reduce the froth produced by boiling.  
• Candy may include non-kosher oil that is put into the molds so the candy does not stick.
• Kosher meat might not be kosher for Passover.

Transference of Taste (Ta'am)

Sometimes dairy will spatter onto a meat utensil, or someone will set a hot pot of kosher food into a non-kosher sink.  Or someone will cut a lemon or onion with a dairy knife and then put the lemon into a pot used for meat.  What happens next depends on whether the offending substance was:

1. Charif (spicy/sour/strong) enough to transfer the taste to the new item.
2. Hotter than yad soledet bo (too hot to hold your hand in it for a few seconds—about 120° F, or 49° C).
3. More than 1/60th of the total volume.

See following halachot for what to do next.

Kitchen Set Up

A hungry Martian landing in a modern kosher kitchen must assume earthlings eat in binary: Ideally, two sinks. Two dish towels.  Two sponges.  Two dishpans. Two cutting boards.  Even, if the owner is fortunate, two dishwashers.

And what about those strange markings on the pots, pans, and servers?  Perhaps he'll find a bright splotch of red paint or an “F” (for fleishig--Yiddish for “meat”) lettered in nail polish on utensils in the left cabinets.  Blue paint or nail polish, or an “M” (for milchig--Yiddish for milk) on utensils in the right cabinets. The plates, bowls, and silverware in left cabinets do not in any way match those in the right cabinets. Somewhere in a central cabinet, pots, pans, and servers are painted with a white dot, marked with a “P” for pareve, or left unmarked.

Opening the pantry, little symbols jump out from canned and packaged goods.  Star-K, O-U, O-K, KOF K…..  Only the dried beans and grains seem symbol-less.  And the freezer?  Well stocked but no frozen bacon, pepperoni pizza, and shellfish TV dinners.…

How do these people eat?

The Great Divide

Separating Dairy and Meat

Welcome to the world of dairy and meat. Most kashrut problems in the kitchen involve the transfer of milk or meat flavor to the other gender by means of heat or, less commonly, by hot/spiciness.

It's easy to be jealous of vegetarians, or people who only eat plants and dairy products or who only eat plants and meat products!  They never confuse their pots and serving utensils or deal with spatters of hot dairy foods onto meat utensils or vice versa.  Large institutions and kosher cafeterias, similarly, may not have these mix-ups, since they can usually devote a whole room to a dairy or a meat kitchen.

Here's how the rest of us live:

Countertops

If you can, designate some countertops for dairy and some for meat.  This will help you stay organized spatially.  If you have only one sink, you may need to use the counter to the left for one dishrack (dairy or meat) and the counter to the right for your other dishrack. 

Some countertop materials, such as granite, can be kashered by pouring boiling water over them.  This will make the counters kosher and pareve (neutral--not dairy or meat).  Once you have kashered your counter(s), you will be able to set down hot utensils, pots, and pans directly onto the counter (dairy utensils on your designated dairy counter; meat utensils on your designated meat counter). 

If your countertop is not kosher or kasherable, you will need to cover the countertop before setting down hot (above 120° F) utensils, pots, and pans. Trivets work fine but so does a simple piece of corrugated cardboard in a pinch.  

Dishes and Flatware

If feasible, select different patterns of dishes and flatware for dairy and meat so you can tell them apart.  It is helpful to store the dairy and meat dishes in separate locations, preferably close to the counter of its gender. Porous dishes (stoneware, china, ...) cannot be kashered once used for hot non-kosher food and cannot be changed from one gender to the other. Metal dishes generally can be kashered. Glass only assumes a gender if it is placed directly on a fire or other heat source (to at least boiling temperature) or into a hot oven, so even if you pour boiling water or hot food into a glass bowl, such as hot pasta, and add cheese or other dairy food, the bowl remains pareve (or whichever gender it was previously).

Sinks and Dishracks

If you don't have two sinks--one for dairy and one for meat--and must use the same sink for both, try to choose different colors for your dairy, meat, and pareve dishpans, dishracks, and sponges/scrubbers (or sponge holders). If not, distinguish your dairy dishpans, dishracks, and sponges/scrubbers (or sponge holders) from your meat ones by placing them on opposite sides of the sink. Neutral, or pareve, dishes/cookware require a third sponge and dishpan. In a pinch, you can wash dishes, pots, and utensils by holding them in the air or placing them on a counter (whether either kashered or not) next to the sink as long as the dishware, pots, etc., do not reach 120° F.  

Drawers

You can designate one drawer for dairy flatware and a second drawer for meat (and a third drawer for pareve). Color-coding or purchasing “dairy” and “meat” stickers to place on the outsides of cabinets and drawers can be especially helpful if anyone else will be cooking/washing dishes in your house and doesn't know your kitchen well.  

Cooking Utensils/Food Processors

Distinguish your cooking utensils (your choice of colors) for dairy, meat, or pareve by using paint or nail polish, using different patterns, or even different shapes (one person uses round baking dishes for dairy and rectangular ones for meat!). If you lack drawer space, hang utensils from the wall or overhead rack or put them on your counter in jars color-coded for dairy, meat, or pareve. In a pinch, colored electrical tape can be used temporarily to mark dairy or meat servers or serving pieces (until it falls off during washing or turns black in the oven…).

You will only need one blender, blending stick, bread machine, mixer, food processor, etc., if you always keep them pareve.  Otherwise, you may need duplicates of these items. Color-code them as well.

Stove Burners

To kasher a non-kosher stove burner, clean off any hard deposits on the grate, cover the burner with a sheet of metal (to hold the heat on the grate), and heat it full-blast for 45 minutes. (See halachot below for kashering burners by putting them in the oven.) 

Stovetop

A stainless steel stovetop can be kashered, but a ceramic one (due to porousness) might not be kasherable-consult a rabbi.  When cooking, place an appropriate spoon rest or bowl nearby (for dairy or meat, depending on what you are cooking) to hold your hot stirring spoon or spatula. This way, you won't need to set down your hot stirring utensil onto a non-kosher countertop or stovetop, or place a hot dairy stirrer where you previously set down a hot meat spatula.  

Oven

You can kasher a non-kosher oven by cleaning off any accumulation of old food (whether burned on or not, it must be removed) and turning up the oven full blast for 40 minutes.  You may use the same oven for dairy and meat foods if you always keep either the dairy or meat covered. Consider the oven to be one gender and always cover liquid foods of the opposite gender (dry foods do not require a cover).

Cutting Board

If you only have one cutting board for fruits and vegetables and one knife, you may want to keep them pareve. The main kosher problems with knives and cutting boards happen when cutting a fruit or vegetable with a strong-spicy taste that can transfer the milk or meat status of one utensil or food to another.  Such items are garlic, lemon, onion, and sour apples, and sour grapefruits.

Examples

• Garlic was chopped with meat knife on a dairy cutting board (rendering the garlic, the knife, and cutting board non-kosher), or
• Onions cut with a dairy knife were tossed into a boiling meat pot (rendering the pot and contents non-kosher unless the onions were less than 1/60th the volume of the pot's food). 

TABLE'S SET

Glasses, washed, can be used for a dairy or meat meal. You can use the same salt and pepper shakers and clean glasses for dairy and meat; however, it is recommended to use separate salt and pepper shakers since you might have food of one gender on your hands when you use the shakers of the opposite gender. If you typically use a table for serving either dairy or meat, and want to serve the opposite without switching tablecloths, lift the tablecloth and use the original table surface or cover the tablecloth with placemats. If one person wants to eat dairy and another wants to eat meat at the same time on the same table, place a reminder to remind them not to mix the foods (different placemats or tablecloths, physical barrier between the people's dishes, etc.).

COOKING FOR RELIGIOUS JEWISH FRIENDS

Let's say you don't keep kosher and want to have your kosher-observant friend over. What to serve?

As long as your utensils are clean, you chose kosher foods (see Going Shopping, below) or fresh fruits and vegetables, nothing gets 120° F or above, there is no involvement of anything spicy (charif), and you don't mix dairy and meat (don't offer a kosher bologna sandwich with kosher Swiss cheese!), everything should be OK. Some people will prefer if you serve them using disposable plates, bowls, flatware, and cups; if you are Jewish, you should only serve on disposables. Some will prefer to be in the kitchen during food preparation. Don't be offended; it's hard to keep track of everything to remember even in a kitchen set up for being kosher!

You might want to keep the wrappers or containers from any processed food so that the kosher guest can see what you actually are serving and check for the ingredients or for a kosher supervision symbol.

GOING SHOPPING

Major towns usually have at least one kosher supermarket, but you can find plenty of kosher food in regular supermarkets too. (Even in Salt Lake City, home of the Mormons, a major supermarket chain sells Empire Kosher Chickens!) Here are some tips:

• You may consider all fresh and uncut fruits and vegetables to be kosher. Sharp-flavored fruits and vegetables such as garlic, when cut, must be cut with a kosher utensil.
• Look for a kosher symbol (“hechsher”) on prepared foods (except those foods that do not need a hechsher—see When Hechsher Needed  and When Hechsher NOT Needed).

For more information on kosher symbols and on what goes into certifying a prepared food as kosher, see this link: http://kosherquest.org/symbols.php

WHY EAT ONLY KOSHER?

The basic reason that Jews only eat kosher food is because God commanded us to do so. There are many explanations of how eating kosher benefits us. One approach is that kosher food enhances the spiritual well being of the Jewish people. That holiness is blocked when we eat non-kosher.

While kosher food raises us up spiritually, we raise it up too. When we say the correct blessing before or after we eat, we acknowledge that God is the food's true source. When we use food's resulting health and strength to perform God's commandments, we reunite our food and ourselves with our higher purposes, “rectifying the world.” That brings spiritual and physical blessing down to us and to the world.

You don't want a rapacious spirit?  Don't eat predators. You don't want to think like a bottom-feeder? Don't eat scavengers—whether catfish or vultures or pigs—or reptiles, amphibians, or bugs (except kosher grasshoppers!). You don't want to be callous? Don't eat the life-blood of a bird or mammal—or even the bloodspot of an egg. You don't want to be cruel? Make sure the animals you eat were slaughtered quickly and humanely. Don't want to separate yourself from worshipping the Only One? Don't drink wine or grape juice that could have been used for idol worship.

And non-Jews? Shouldn't they keep kosher too?

Non-Jews must keep only one kosher law--aver min ha'chai. This means non-Jews, like Jews, may not cut off and eat the limb of a live animal.

We can come up with numerous explanations for why keeping kosher is healthier, more pleasant, more logical, or more spiritual than eating non-kosher. But the bottom line is, we do it because God says to, we are here to serve Him, and we trust that God wants what is best for us!

It is a custom for fathers to cover their sons' heads with their talit while the priests/cohanim bless the congregation.

The Torah forbids cooking on Shabbat.   “Cooking” means making food edible by heating it to above 120° F (49° C).
Cooking includes:

• You may not make a soft food hard (such as cooking an egg).
• You may not make a hard food soft (such as cooking meat).
• You may not, in any manner, heat (to 120° F or above ) liquids that you will drink or foods with liquids—such as sauces and gravies--whether fully cooked or not. 

According to the Torah, you may eat food on Shabbat that had been placed on the heat source Friday afternoon but was not completely cooked by sunset.
ReasonNo action is being taken and the cooking will be completed by itself.
 
According to Torah law, food is only considered to be cooked if it has been directly heated from the heat source, such as a kettle on a fire or a pot on a flame (even if that utensil has been removed from its heat source). This is called a kli rishon.
Once you pour water from a kli rishon into a glass, the glass is a kli sheni. Some foods, such as an egg or tea, get cooked in a kli sheni (kalei bishul--easily cooked). These foods are forbidden by Torah law to be put into a hot kli sheni on Shabbat.

• You must have taken it off with the intention of replacing it, and
• You may not put the utensil down onto a surface; you must continue to hold the food (or the utensil) in your hand.
• The heat source must be covered.