What does it mean when a girl keeps glancing at my lips when I'm talking?

I know that making eye contact is, most of the time, a sign of politeness. However, a lot of times when I'm talking to this girl (most of the time casually, or having a discussion) we maintain eye contact for a long time, or at least it seems that way. The eye contact is so prolonged that I kinda look into then and try to see if the get bigger when she looks at me, the fact that I have time to do that means that we really look at each other a lot. So with the CONSTANT eye contact, I can see she breaks it for a second or two to look at my lips when I'm talking to her. Until now, I noticed that I do the same thing, I look at her lips and her teeth (I know I like her, so I'm assuming that's why). Is it normal for people to do that? Its like I know her mouth so well and I don't remember anybody else's mouth at all hahaha. Thanks for the answers.
What does it mean when a girl keeps glancing at my lips when I'm talking?
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