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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 when they are mispronounced, while others simply rely on the confusion and mistakes of the speaker for their amusement value.
Betty bought some butter, but the butter Betty bought was bitter,
so Betty bought some better butter, and the better butter Betty bought was better than the bitter butter Betty bought before!