The short answer is that cottage cheese is made with milk and rennet. Rennet is an enzyme that helps to coagulate the milk, creating the solid curds that are characteristic of cottage cheese. However, there are a few different types of rennet, and the type used can affect the final flavor and texture of the cottage cheese. For example, vegetable rennet may be used to create a more mild-tasting product, while animal rennet can result in a stronger flavor. The liquid component of cottage cheese is known as whey. Whey is the watery part of milk that remains after the solid curds have been separated out. It contains protein, vitamins, and minerals, as well as lactose (milk sugar).