What does it mean a guys lip part when looking at you?

While we was maintaining eye contact it seemed very focused on both sides becauze he was sat next to his friend and his FULL attention was on my eyes 👀

So basically we maintained eye contact while i was walking past and when it got to opp him his lips parted a bit and his head moved aligned to my walk

I never had this happen before ☺️

What does it mean a guys lip part when looking at you?
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