Basically, tell a joke. Rules:
No Yo Mama Jokes, because they're a form of banned forum games.
No Knock Knock Jokes, for the same reason.
Do not include another PureZC member in your joke, so as to keep the flaming down. Of course, if they say they're fine with that, then you may.
Try not to flame.
Try not to repeat a joke.
Any NSFW jokes or jokes not suitable for children (sexual theme, drugs, excessive violence) should be spoilered, and the reason you're classing it as NSFW/inappropriate should clearly be stated outside the spoiler.
Answers to question jokes/one-liners should preferably be hidden, though you don't have to do so.
Any jokes based on a subject many people are touchy about (racist jokes, jokes about WWII) are out. Black Comedy (jokes about death, not about black people, specifically) is allowed, but if enough people show dislike for it, then I guess we'll have to remove it.
Disclaimer: Any joke based on a specific person/group of people (Scotsman, Englishman, Irishman Jokes, Viola Jokes etc.) listed here are not a direct insult to those groups of people. If you belong to any of the groups in the joke, you are not being targeted, and please do not take offence.