This one is brilliant! My wife even hesitated to tell me because she realises how silly it is!
When he woke up the other day, our baby son had some hair on the back of his head stuck together.
"Oh dear", exclaimed my wife. "He's been visited by dwarves and they've been playing with his hair and twisting it together!"
I just cracked up. That's the funniest one I've ever heard!
My previous favourite was "if you go to bed with wet hair, you'll wake up blind", but this one is much better!