Why do i always need a pen in my hand?

whilst im at school i always feel the need to have SOMETHING in my hands.... usually a pen... preferably a clicky one. without one i cant concentrate properly and tend to just shake my leg and look around something, but even with the pen in my hand I absolutely HAVE to click and dismantle it and that sometimes helps me concentrate and listen in lesson, however my teachers think otherwise and it generally annoys them. also when i have the pen in my hand i always feel the need to doodle and draw on what ever i can.... i dont know why i do this, i just cant concentrate and get extremely bored if i dont