Whey is a watery liquid that comes out of milk when it has been curdled and strained. It is also called milk plasma. It is a by-product of cheese making. Sweet whey is made during the making of rennet types of hard cheese like Cheddar or Swiss. Acid whey (also known as "sour whey") is made during the making of acid types of cheese such as cottage cheese. Whey also forms as a liquid on the top of yoghurt.