Why is this woman staring at me continuously despite her talking to another guy?

She’s in my friends group at work and I notice that she keeps staring at me when she thinks I’m not looking.( I notice and it’s obvious ). I’m also a pretty fit guy. I lift weights regularly. Also, We do talk sometimes, but I always talk to her as a friend. I make her laugh as well but I don’t even flirt with her. I don't know why she keeps staring if she’s in a deep talking stage with another guy at work.

Why is this woman staring at me continuously despite her talking to another guy?
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