Thinking about yawning, or seeing someone yawn, can make you yawn. So can listening to an audiotape of a yawn. And just reading this article could set you off... but we don't know why.

Research suggests that we yawn when our state of alertness is changing. We know it isn't to get more oxygen - lungs can't sense oxygen levels, and pre-born's yawn even though their lungs aren't ventilated. Scientists think that yawning is a reflexive throwback to prehistoric times.

Extreme yawning can place stress on your jaws, causing them to lock or to become sore. Yawning can trigger symptoms of Temporomandibular Joint Disorder (TMD), so if you suffer from TMD, just don't open too wide! And watch out... yawning is contagious!