A yawn is an involuntary reflex where the mouth is opened wide, and the lungs take in a lot of air. The air is then exhaled slowly. During this time, the eardrums stretch, and the eyes may also close tight, causing them to water.
No thought or action has to be taken to produce a yawn, and the process is similar for everyone. Yawning commonly occurs either before or after sleep, which is why it is usually considered a sign of being tired. Yawning also occurs frequently in people who are doing boring or tedious things.
There is also a social aspect to yawning. Yawning appears to be contagious among humans and other animals, and the contagiousness of a yawn is well documented but hardly understood.