I remember taking great pains to teach my daughter how to tell a joke, so much fun in itself. When she wakes, I'll ask her which it was, she'll remember.

My favorites:

Knock-Knock
Who's there?
Canteloupe!
Canteloupe Who?
Canteloupe tonight, Dad's got the car!


Say Knock-Knock.
Knock-Knock!
Who's there?
... (any number of things might ensue, but the look is always priceless)

Dani