Why did my classmate look at my breasts when I was talking to him?

today i was wearing a tank top that wasn't even that revealing and i dont have like large breasts but my classmate who never showed any interest and we barely talked kept glancing at my chest when i was talking to him. I dont know if he thought i wouldn't notice because he took glaces several times. When i told my friend she told me that all the guys do it and that the same thing happened to her with one of our closest guy friends. So do guys usually do this?

Why did my classmate look at my breasts when I was talking to him?
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