Why do we yawn?

[matt_london1]

and why is it catching?

[rodzd126]

is a reflex of deep inhalation and exhalation associated with being tired, with a need to sleep, or from lack of stimulation.

[jemimaC]

Now you've started me off!

[Veronica_007]

The reason why we yawn is because of the lack of oxygen in the body being the reason why you take a huge deep breath in and it is catching because it is a very suggestive thing and people whoi are easy influenced will repeat it

[Muppit]

Wrong wrong wrong wrong wrong!

We yawn to equalise the pressure difference between the outside and inside of our ear drums. As we yawn, we change the pressure within the room we are in, thus changing the pressure difference for everybody else in the room. So gthey yawn to equalise their pressure differences.

By the time that two or three people have yawned, the pressure differences are negligable and so no one else will need to yawn. You will also find that people who were luck enough to have gromits installed, or perforated ear drums will not yawn. If you do see one of these people yawning, then they are so faking it. Much like this answer.