As they say, you are what you eat. But in the case of protein, the old saying rings true. Our skin, our bones, even our hair and fingernails are composed almost entirely of protein. And perhaps, most importantly, protein fuels the muscle-growing process. But for as common as protein is, there's a lot to understand about how it works and the ways in which we can reap the most benefits from this all-important nutrient. Want to top off your knowledge? Here's what you need to know.