Yaaawwwnn...!

Do you know why yawning is contagious?
Or how can we know whether the human population on earth is odd or even?

Well, the answers to both these questions are inter-linked.

You see, when a person yawns, he blows out some air, and this creates a slight increase in pressure in the atmosphere. This in turn prompts another person located nearby to take an intake of breath to balance it, and hence he yawns. Neutralised.

But then, if there are odd number of people on earth, one person would not find anyone who can neutralise his yawn, and so he will have to do it himself, and he will keep on yawning. Hence you can figure out if there are odd number or even number of people on earth.

Or it could just be that you are yawning because you're bored and sleepy.
Seems a much easier explanation to me!!