What does it mean when I keep catching a girl checking me out or staring. help please?

A girl at work is very attractive and we like to flirt. Sometimes I catch her staring and when I look at her she quickly looks down. Other times I see her look at me top to bottom very quickly. Sometimes she does it very slowly to the point where it's easy to notice.

Why do girls check out guys and what is happening here?
What does it mean when I keep catching a girl checking me out or staring. help please?
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