Guys, Why do men lift their shirts and expose their stomach while talking to a woman?

There is a guy I've had classes with for about a year. I'm 10 years older than he is (but look his age) and find him attractive and intelligent. For the last 2 months or so he has started lifting his shirt and leaning back stretching so his shirt comes waaay up. I don't want to misinterpret the behavior, so I'm curious for feedback from other guys. Thanks!
Guys, Why do men lift their shirts and expose their stomach while talking to a woman?
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