Girl open her mouth slightly right after eye contact?

Hi, So what is this about? It happens rather frequently, either she opens her mouth slightly or pulls her lips inward. This happens a lot, and I don't really get why. No smile or any other positive signs, just extended eye contact and the open mouth thing. And it's not a jaw drop or anything, just a slightly open mouth. Have you found yourself doing this, and if so in what situation and why? And I only started focusing on it because it happen rather frequently and I found it somewhat strange.
Girl open her mouth slightly right after eye contact?
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