Nobody really knows why humans or animals yawn. In fact, there is not much we know about yawning. We do know that yawning is an involuntary reflex. People often yawn when they are tired or sleepy, but this is not always the case. If you take a deep breath just before you feel you are about to yawn, you can stop a yawn from happening. It is often thought that yawning is contagious but this has not been scientifically proven. Some people believe that we yawn when the brain is not getting enough oxygen. Sea turtles have been seen yawning underwater!