What was the last joke you heard/read that wasn't funny? Preferably the really unfunny ones, not the ones that are 'so bad they're good'. The ones that are followed by an uncomfortable silence and then one of those awful almost-laughs.
Knock Knock. Who's there? Tennis. Tennis who? Tennis five plus five.
ETA: Okay, some of you find it funny. :D I didn't at the time, though usually it's the sort of thing I would laugh at.