Friendship
 

Friendship among women: one doesn't come home one night, and tells her b\f that she spent the night with a female friend of hers. B\f calls 10 of her friends, and none know a thing.

Friendship amongst men: Same thing happens. Man says he spent the night at a friends place. G\f calls 10 of his friends. 8 confirm he has been there, and the two others say he's still there.

That's a good one, saw it on bash.org recently.