Butter is made up almost entirely of the fat in milk. Churning causes the globules of fat to separate from the rest of the milk and cling together as butter. Cheese contains both the butter-fat and the casein, which in buttermaking remains in the butter-milk. To make cheese, milk is curdled by adding rennet to it, and the curds are separated from the watery whey. After being allowed to "ripen" till partly sour, the curds are salted, placed in cloths in molds, and squeezed by a press into cheese. Soft and " cream " cheeses are not pressed.