Why is yawning contagious?

Can you get to this question without having to yawn?

Yawning is contagious to both children and adults. Even some animals, like dogs, can yawn! One study of adults showed that yawning becomes less contagious with age. Children younger than four years old and children with autism spectrum disorders may be less likely to yawn when they see others do so. There are many theories why yawning is contagious. One possibility is that it helps synchronize people within the group, by indicating that it's bedtime, for example. Another suggests that it helps regulate our brain temperature. It may also be a sign of empathy — although not all studies support this idea.