Quirk #1: Why do you laugh until you cry?

Experts don't really know. One thing to consider: Laughing and crying are similar psychological reactions. "Both occur during states of high emotional arousal, involve lingering effects, and don't cleanly turn on and off," says Robert R. Provine, PhD, a psychologistat the University of Maryland, Baltimore County, and author of Laughter: A Scientific Investigation.

We associate crying with sadness, but tearing up is an even more complex human response. Tears are triggered by a variety of emotions—"by pain, sadness, and in some cases even extreme mirth. It's just the way we've evolved," says Lee Duffner, MD, a professor of ophthalmology at the University of Miami's Bascom Palmer Eye Institute.



As it turns out, that's good, because both laughter and crying can ease a stressful experience, probably by counteracting the effects of cortisol and adrenaline. So if you ever find yourself laughing until you cry, count yourself lucky. (Get the giggles with these four ways to laugh more.)