In some religions, an unclean animal is an animal whose consumption or handling is taboo. According to these religions, persons who handle such animals may need to ritually purify themselves to get rid of their uncleanliness. Judaism In Judaism, the concept of "impure animals" plays a prominent role in the Kashrut, the part of Jewish law that specifies which foods are allowed (kosher) or forbidden to Jews. These laws are based upon the Books of Leviticus and Deuteronomy of the Torah and in the extensive body of rabbinical commentaries (the Talmud). The concept of unclean animals is also mentioned in the Book of Genesis, when Noah is instructed to bring into the Ark all sorts "of pure beasts, and of beasts that are impure, and of fowls, and of every thing that creepeth upon the earth". In the Torah, some animals are explicitly named as pure or impure, while others are classified by anatomical characteristics or other criteria. In some cases, there is some doubt as to the precise meaning of the Biblical Hebrew animal name. According to Jewish dietary laws, to be "pure" an animal must also be free from certain defects and must be slaughtered and cleaned according to specific regulations (Shechita). Any product of an impure or improperly slaughtered animal is also non-kosher. Animal gelatin, for example, has been avoided, although recently kosher gelatin (from cows or from fish prepared according to kosher regulations) has become available.; the status of shellac is controversial. The prohibitions also extend to certain parts of pure animals, such as blood, certain fat tissues, and the sciatic nerves. Finally, it is forbidden to cook the meat of an animal in the milk or dairy product of that same animal, which has in turn led to the traditional practice of using separate complete sets of kitchen utensils for meat and dairy so as to totally ensure this rule is not broken. Classification of animals The Torah does not classify animals under modern scientific categories of mammals, fish, reptiles and birds. Rather, the religious categories are land-dwelling animals (land mammals, flightless birds, and land reptiles, etc.), flying animals (birds, insects, flying mammals such as bats), and given that each of these religious categories of animals includes species of at least two or more of each scientific categories of animals, there is no general kashrut rules relating per se to mammals, birds, reptiles, or fish.