Why do guys stare at girls?

Why do guys stare at girls? I know it sounds stupid and its because they are nice to look at duh! but I'm talking repeatedly staring, not checking out but when a guy is watching you, you look at him, he keeps staring/he looks away, you look back again and he is still staring. Apart from liking a girl what actually makes you do it?
Why do guys stare at girls?
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