A protein shake is a beverage made by mixing protein powder with water, milk, or other liquids. It is often consumed by athletes and fitness enthusiasts to help build muscle, recover after workouts, or supplement their diet. Common sources of protein in these shakes include whey, casein, soy, and pea protein. Protein shakes can also include additional ingredients like fruits, vegetables, or nut butters for added flavor and nutrition. They are convenient and can be made quickly, making them a popular choice for those looking to increase their protein intake without preparing a full meal.