Huge smiles.
I'm not talking grins here, or regular smiles; no, I'm referring to those politician and model smiles, ones in which the people doing the smiling seem to grow extra teeth just for the occasion. I honestly don't understand how people do that. Their lips spread and spread, more and more teeth appear, and still they are clearly, if sometimes obviously forcibly, smiling.
When I try to smile like that, I look positively deranged. My expression changes from its normal neutral state, which many folks have characterized as "I might just kill you," to a positively insane "I'm going to kill you, rip your skin off in one pull, and gnaw on your bloody bones while abusing myself" sort of look. Unless you're trying to get a seat on a crowded bus or give small children nightmares for weeks, this is not a good face to deploy. It worries people.
I clearly missed the necessary lessons, because I don't understand how these people manage such huge smiles. I expect I never will.