Despite many believing it was only a myth, yawning is contagious. Yawning itself is your bodys way of telling you that you need more oxygen. It does not in fact have to do with being tired although it is associated with it. You could be wide awake and yawn just because you need more oxygen. the common belief behing contagious yawning is mirror neurons. For those of you who have taken psychology you should understand this concept, for those who havent, let me explain. Mirror neurons have to do with the saying "monkey see, monkey do". Throughout development these mirror neurons allow you to watch someone do something and then repeat it ourselves. So when seeing someone yawn, the mirror neurons inside of your brain become active and actually replicate the same behavior causing you to yawn.
If you were to look at this photo long enough you may feel a yawn coming on yourself.
Yawns are in fact contagious, but you don't always yawn because you're tired, but because your body needs more oxygen.