Why does he cover his mouth while staring at me?

This is kind of a weird question, but there's this guy in one of my classes who I'll catch staring at me from across the room. And he usually has his hand covering his mouth completely. Like sideways, with his left hand/palm covering his lips. I tried to kind of smile at him the other day, but it's kind of awkward smiling at someone who isn't showing their mouth!
Why does he cover his mouth while staring at me?
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