Take the one that the guy tells in 28 Days Later!
"A man and a giraffe walk into a bar. They both get pissed. The giraffe falls over. The man goes to leave and the bartender says, "Oi, you can't leave that lyin' there!" And the man says, "No, that's not a lion. That's a giraffe."
Corny, but it should get you some laughs.