Traditional ricotta is made from whey. Whey is the liquid that is pressed out of cheese curds when cheese is made. Instead of discarding the whey, Italian cheesemakers discovered they could re-heat it and the whey would thicken into curds. This is how ricotta (which means recooked in Italian) got its name. These days, many brands of ricotta are made by combining whey with whole milk or cream. You can even make a relatively simple version of homemade ricotta cheese using whole milk and vinegar or lemon juice.