Why do teen boys stick their hands down their pants?

Loads of the boys in my school will just be sitting in cl or walking in the corridor and then just shove their hands down their pants. They don't care who is about or what. But what i have noticed is that the "popular" guy's don't do it...until today when i saw the fittest lad in my year and school just shove his hands down his pants. Lol. What are they actually doing? :/ :S x