My son does that. He's a comedian to boot... so he has a whole RANGE of forced/fake smiles. Sigh.
The GREAT thing is that a good photographer takes so many pictures and talks so much that people (even kids) relax and start using real smiles. School photographers can't do this (3 seconds, boom, next), and as a mom I rarely have time or patience... so I catch him candid all the time. I think my fave this year is him on a treebranch. Although some of my favorites ALSO include times when I DID have the time and kiddo ran out of patience with ME and "charged" the camera. Lion growling faces are so rarely caught on film, that I find them just as precious as that true *great* smile. I mean, temper tantrums are annoying at the time... but looking back on toddler years before I had a great digital camera, there are a lot of moments I only have in my memories.
Digital rocks my world. I only get about 1 good shot in 50... so I have tons of good one of him. Could NEVER have afforded to do that in film.