Pictures with teeth in them are more interesting than pictures without teeth in them: human, animal, or otherwise. Cigarettes are also good go-to's, especially the lighting of them. Hell, anything on fire is pretty interesting, add a mouth to the equation and you are golden. Vibrant real colors and awkward fleeting expressions. Life is always better than a staged scene, multiplied by it's obliviousness.

If it is still there after you click the shutter is it worth your time?