Watching someone yawn makes you more likely to yawn too

Contagious yawning shows up not only when you see someone yawn, but often just from reading about yawning, as you may be experiencing right now. Psychologist Robert Provine's experiments found roughly half of people yawned within minutes of watching a video of others yawning, while a control video produced almost none. It isn't unique to humans either — chimpanzees, dogs and even some fish have been shown to catch yawns.

— Robert Provine, Yawning — Bulletin of the Psychonomic Society, 1986