Rennet is a set of enzymes, chiefly chymosin, drawn from the fourth stomach (abomasum) of young ruminants such as calves, goats, or sheep, that coagulates milk protein into curds separate from the liquid whey. It's the basis of most hard and semi-hard cheeses, including cheddar, Parmesan, and Emmental, and some soft ones. Rennet's use is thought to have been discovered by accident when milk was stored in vessels made from animal stomachs; the earliest solid archaeological evidence of cheesemaking, sieves recovered in Kuyavia, Poland, dates to roughly 5,500 BCE, though it doesn't confirm rennet specifically was already in use that early. Vegetarian, kosher, and halal cheeses use microbial or plant-derived coagulants (such as thistle or fig sap) instead of animal rennet.