Guy's Behavior

Why did he grin at me like that?

I was sitting in a hall and this guy in front of me turned around and stared at me for a couple seconds.

He looked at my face for longer than would be normal for sure. He then seemed to look at what I was wearing because I saw his eyes go towards my shirt.

Then he grinned and turned back around.

The whole time I kinda acted like I didn't notice him because I would have felt weird looking at him directly. I kinda noticed all this from my peripheral vision.

Is this something guys do when they are attracted to a girl or something? Or is he just weird?
Why did he grin at me like that?
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