When girls constantly look at a guy's lips?

I've noticed some girls look at my lips when I'm talking to them. I'm sure some of them just do it without it meaning anything special, but there's sometimes when I feel there's something behind it. For instance, there was this girl who I was really into and when I talked to her, she would look at my lips every 5 or 10 seconds and then back into my eyes. What does it mean if it happens more than once or twice during a conversation with a girl?
When girls constantly look at a guy's lips?
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