I am suprised nobody mentioned this Bert and Ernie sketch:
Ernie counts sheep, fire engines and balloons -
Ernie can't sleep, so Bert tells him to count sheep. Ernie tries to count sheep, but it's dull -- so Ernie starts counting fire engines. The noise from the imaginary fire engine wakes Bert up. So Bert tells Ernie to count something quieter. So Ernie counts a balloon -- but the balloon gets bigger and bigger, finally bursting with a huge explosion that wakes Bert.
This sketch makes me laugh everytime; especially Bert's reactions to Ernie's imagination.
Too funny!