Yawning

Hey guys!
Sorry for the lack of updates recently. This past week I worked a 12 hour day and a 15 hour day, so I've been working and sleeping a lot! One thing I have learned from being on set this much is that yawning is, in fact, contageious. Last week I passed yawns back and forth with Ms. Sophia Bush and yesterday I literally sat for 15 minutes and continually passed a yawn back and forth with one of the crew guys. Yawning is like a terrible game of ping pong that just goes back and forth and back and forth. Why? What is the science behind that? Do tell!