Oh boy, I am trying to reply again, I spent a lot of time trying to reply and it didn't go thru.

Relax, I used to twittle my hair and bite my nails. It was my brain's way of trying to sooth the stress that I was feeling. The next time you are in a classroom or public place look around at the other people in the room.
Look at what their bodies are doing. Some of them twitch their foot or shake their foot back and forth, some of them make their knees weave in and out, some of them take their pencil and shake it back and forth and look at the pens and pencils they have in their hands, many of them are chewed or bent. Some peope pull on their noses or hold their chins. Look at the hands of other people you may see bitten nails or knawed on fingers.

Everyone has something that they do when they are stressed. All these things are ways in which our brains try to find a self soothing way of controling stress.