When I was playing sports in school I don't think anyone ever mentioned the word concussion to us. Subsequently, I'm sure that one or more of my classmates had undiagnosed concussions. Fortunately, that is not the case for today's student-athletes. Today, we hear about concussions in professional and amateur athletes quite frequently. But what is a concussion and how do they happen? Dr. Sanjay Gupta explains the answers to those questions and more in the episode of CNN's Explain It To Me series embedded below.