Why are yawns catching?

[private joker]

Whenever I see someone yawn, I start yawning. Sometimes I'm just talking or thinking about yawning, and I yawn.

Just recently I was looking on Google images for a funny shot of someone yawning, and I started yawning while looking at the pictures of people yawning.

It must be something in how the brain communicates to the lungs or whatever, but could any med students or psychologists help explain this weird phenomenon? I can't be the only person who finds yawns contagious.