This is the age old way to get a smile out of a person about to be photographed. The other way of course is say something funny.
I like photos that make you laugh though, or even just smile and one of the best photographers I seen for this has to be talented photojournalist Paul Treacy. He has just started a new blog called photohumorist go and have a look it will put a smile on your face.
The photo above is my son hamming it up for the camera. I never need to say cheese to him.