What's brown and sticky?
A stick!
What did the fish say when he ran into the concrete wall?
Dam!
This is one of my favorite old Bert and Ernie routines, and my 4-year-old thinks it's the funniest thing ever:
Say, "We're going to play a game. I'll say 1-Q, then you say 2-Q." Go on in that manner, alternating until the child says 10-Q, at which point you say:
"You're Welcome!"
This one for older kids who've been through music class or piano lessons:
Why couldn't Mozart find his teacher?
Because he was Haydn!