He does this all the time to elicit laughs out of people when we're out and about. It is so funny! He squeezes his hands real tight and makes this face. He does it over and over again. We'll be at the grocery store and he'll do it (unprompted) for the cashier. I never taught him this face, but after he did it the first few times, I did encourage it.
I think I have myself a little extrovert. :)