Interestingly, while all vertebrates (including fish) yawn, only humans, chimps and possibly dogs find yawns contagious. And people don't find them contagious until they're about 4 years old. Recent studies show contagious yawning may be linked to one's capacity for empathy [source: Sohn].
In one study, autistic and non-autistic children were shown videos of people yawning and people simply moving their mouths. Both groups of kids yawned the same amount when viewing the video of people moving their mouths. But the non-autistic kids yawned much more frequently than those with autism when watching people really yawning. Since autism is a disorder that affects a person's social interaction skills, including the ability to empathize with others, the autistic kids' lack of yawning when watching others do so could indicate they're less empathetic. The study also found the more severe a child's autism, the less likely he or she was to yawn. On a positive note, someday doctors may be able to diagnose cognitive disabilities in young children more easily by seeing whether or not they can catch a yawn from others [sources: Kahn, Sohn].
So even though we still don't know for sure why we yawn, we do know lots of interesting things about yawning: you start yawning in utero; you yawn when you're aroused; more than half of you will yawn if you see someone else yawn; and reading about yawning makes you yawn [source: Sohn].
So, how many times did you yawn while reading this article? We hope not too many.