A tongue-twister is a phrase that is designed to be difficult to articulate properly, and can be used as a type of spoken (or sung) word game. Some tongue-twisters produce results which are humorous (or humorously vulgar) when they are mispronounced, while others simply rely on the confusion and mistakes of the speaker for their amusement value.
Have you ever seen a woodchuck? Are they real? Are any of us real? Is a tongue-twister a proper topic for an online-forum?