Whey is the watery liquid that separates from milk when you make cheese or yogurt. If you've ever opened a container of yogurt and seen a thin, slightly yellowish liquid pooling on top, that's whey. When cheesemakers heat and curdle milk to make cheese, the milk separates into solid white chunks called curds and this watery whey.

For thousands of years, people considered whey a worthless byproduct and often threw it away or fed it to pigs. But whey contains protein, vitamins, and minerals that make it surprisingly nutritious. Today, manufacturers dry whey into powder and add it to protein shakes, energy bars, and infant formula. Athletes drink whey protein shakes to help build muscle after workouts.

The nursery rhyme about Little Miss Muffet, who sat eating her “curds and whey,” describes an old-fashioned way of eating fresh cheese with its liquid still mixed in, something like cottage cheese today. While that might sound unappetizing to some people now, separating curds from whey remains the essential first step in making almost every cheese you've ever eaten, from mozzarella to cheddar.