There's no one thing that makes you a “real man,” but your testosterone levels do play a pretty significant role. You could say it's what separates the men from the boys—literally. But how? What is testosterone, really? Why do some body builders sweat their T levels and inject themselves with the hormone or steroids? Is it as important as we've been lead to believe and what happens if you have lower levels than the average man? With all these questions, we thought a primer on T was in order. Herewith, the answers to a few simple questions.