Guys, Why the poker face?

So I was in Physics class and I was doing a lab and my back was facing this guy. I’m not sure what made me turn around but when I did he was sitting close by already staring at me blankly, it was kind of strange. I kind of just smiled and turned around but then I heard him tell his friends something. Why the blank stare? Guys please explain!
Guys, Why the poker face?
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