People having been asking that question for years, and you know what? You've stumped me! I don't know why people yawn, but I'm not alone on that. No one has the real answer, but there are a lot of theories out there.

One hypothesis says that when you're tired or bored, your breathing slows down—so, carbon dioxide builds up in your blood allowing less oxygen to reach the lungs.

The message that you're oxygen-deprived races to your brain, which then sends out a wake-up call to the rest of your body.

In trying to get back in balance, your brain tells you to open your mouth wide to suck more oxygen back in, then has you exhale out that extra carbon dioxide.

Another school of thought calls yawning a type of workout for the lungs—stretching them and keeping them limber.

Or maybe yawning helps distribute a substance called surfactant, which keeps the lungs well-lubricated.

None of these ideas has been proven, but one thing is for sure—yawning is contagious.

Try yawning in a roomful of people and see what happens…you're likely to set off an open-mouthed chain reaction.

Just talking about yawning makes me want to yawn.