How do you smile in a picture?

For some reason I just can't for the life of me. It doesn't matter if it's a family portrait or a picture at a party or just my profile picture here. I just can't smile. I have really good teeth and I smile a lot throughout the day. Hell, people have told me to practice in front of the mirror. But that doesn't help. Is it just a guy thing? Or have other people had this problem before but got over it?
How do you smile in a picture?
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