What is the thought process and reason why women would check out a guy?

When I'm out and about, I'd notice a woman would look at me. I don't understand it. Then again, from what I understand is they could be zoned out. If I'm with my female friend who is my ex-wife, this is when I notice women would look at me. Maybe when I'm at the gym, I think I noticed a woman had look at me.

What is the thought process and reason why women would check out a guy?
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