Quick! Make up a joke!
My friends and I used to play a game where we'd interrupt a conversation or kill dead air by yelling "Quick! Make up a joke!" while pointing at someone. They would immediately have to say a joke without hesitation. The only guideline was that the joke had to be truly novel, not some preexsitent hilarity dwelling deep in the wastelands of her/his mind (or lack thereof). It was really a brilliant game that stretched our improv and comedic abilities. To make a long story short, most of the jokes sucked.