A tongue-twister is a phrase that is designed to be difficult to articulate properly. Most commonly the phrase consists of words beginning with the same or similar sounds or alternates between sounds that are slightly different, thereby making the change from one sound to the next difficult (e.g., The sixth sick sheik’s sixth sheep’s sick). Consequently, they can be a useful exercise for students who have difficulty, for example, distinguishing between /th/ and /f/ or between /sh/ or /ch/.
Early English tongue twisters were developed to teach pupils proper speech and these elocution exercises were practiced as routinely as multiplication tables used to be. To discover the history behind some famous tongue twisters (Peter Piper, How much wood would a woodchuck chuck, Betty Botter bought some butter, She sells seashells, etc.) go to: http://mentalfloss.com/article/513952/history-behind-famous-tongue-twisters
Having students develop their own tongue twisters is a fun way to reinforce graphemes (letters or letter combinations representing a sound) that students are learning, teach the concept of alliteration (when multiple words begin with the same sound) and/or to explore the different graphemes we use to represent sounds in English (e.g., Gentle jolly giant gypsies jumped joyously over gems).
Developing tongue twisters could also be useful for helping students distinguish between homophones and homonyms (e.g., Do you know which witch won one ewe and which one won no ewes?). Illustrating these tongue twisters would be a great extension activity.
For a list of tongue twisters to get your students started: http://www.tongue-twister.net/en.htm
Once students have developed their tongue twisters, you can choose a tongue twister for everyone to say, but each person has to use a different pitch, volume or expression. For example, the first person could say the tongue twister in an extremely happy, laughing voice, but as each student repeats the tongue twister they progressively become either angrier or sadder.