What does it mean if a guy looks at you and bites his lip?

There is this guy in one of my classes and we are pretty close and he knows that i like him and i know he likes me too. But nothing has happened between us since and its bothering me. But the other day looked over at him and he was staring at me and quickly looked away when he saw i was looking and after he looked i saw him biting his lip. My friend saw it to and she freaked out and kinda freaked out and scared me. It happened today as well when we were working on computes and he was sitting next to a girl that likes him and she watched him look at me and bite his lip and she looked kinda mad but we're kinda friends so she got over it really quick it seemed like, but idk. So please help :).