Why does teenage boys stick their hands down their trousers?!?

I have seen a lot of boys doing that in school, even in public! In the exams, the boy who was sitting close to me and he even stuck his hand in there all way through the exam.
Do you think they think it is appropiate?
Why do they do it? (Mostly chavs..)

Update: by the way i am not the one who is giving thumb downs :S

anyway, interesting answers! hope to get more answers ! :)
Update 2: two guys do that in my history lessons!
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