Stephanie McDuff, age 16, of Ottawa, Ontario, Canada, for her question:

WHAT IS YAWNING?

Yawning is an involuntary reflex. When you yawn, you open your mouth wide and take a long, deep breath which you hold for a moment before you breathe out. Often you'll stretch your limbs as you yawn. Once the act has started, it is almost impossible to stop it. You can hold your mouth closed, but the yawning muscles still contract.

You yawn when oxygen is slowly cut off and when the muscles are thoroughly relaxed. The yawn is usually due to drowsiness or fatigue. Yawning improves the flow of blood back to the heart, which will be followed by an increased output of blood and an improved blood supply to the brain.

One yawning person in a group may set the others yawning. This may be due to suggestion or to the fact that everyone is similarly affected by the environment.