For example: this time I looked at this guy that I find really cute because he was looking at me first. when I did looked he made this strange expression in his face, he moved his mouth (maybe he was bitting his lips with his mouth closed, idk... I was far from him) and then looked at his left side.
I wish I knew because I'm guilty of it too. I think it's either because I think she's pretty, but don't want to give the impression that I'm staring. It's the strangest thing because it doesn't always happen, but when it does it sucks getting caught.