Deep eye contact + glancing at lips, what does it mean?

I have been talking to this girl and I have noticed we have a lot of eye contact. Recently I started to REALLY notice her eyes and I noticed that she is looking at my eyes and then quickly tilo my lips and then back up. Its not like we are having an intimate conversation or anything, its just me and her talking and she keeps doing it. What I can tell is that I like her and I do the same because I love looking at her lips, but I don't know if its just a natural thing to most people or what? Thanks!
Deep eye contact + glancing at lips, what does it mean?
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