Bet I can tell your age! called an old lady at an old gentleman walking by. After a couple of denials, the old man asks, how? The old woman says, come behind these bushes and drop trou, and I'll tell you your age. Intrigued, the old guy agrees.

And does.

Hmmm, not sure, please jump up and down? OK...

87! You're 87!

Amazing! How could you tell?

Silly, I was at your birthday party last week!