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. Additionally, they can be used as exercises to improve pronunciation and fluency. Some tongue twisters produce results that 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.

Some tongue twisters rely on rapid alternation between similar but distinct phonemes (e.g., s [s] and sh [ʃ]), combining two different alternation patterns, familiar constructs in loanwords, or other features^[which?] of a spoken language in order to be difficult to articulate. For example, the following sentence was said to be "the most difficult of common English-language tongue twisters" by William Poundstone.

The seething sea ceaseth and thus the seething sea sufficeth us.

These deliberately difficult expressions were popular in the 19th century. The popular "she sells seashells" tongue twister was originally published in 1850 as a diction exercise. The term "tongue twister" was first applied to this kind of expression in 1895.

"She sells seashells" was turned into a popular song in 1908, with words by British songwriter Terry Sullivan and music by Harry Gifford. According to folklore, it was said to be inspired by the life and work of Mary Anning, an early fossil collector. However, there is no evidence that Anning inspired either the tongue twister or the song.

She sells sea-shells by the sea-shore.
The shells she sells are sea-shells, I'm sure.
For if she sells sea-shells by the sea-shore
Then I'm sure she sells sea-shore shells.

Another well-known tongue twister is "Peter Piper":

Peter Piper picked a peck of pickled peppers
A peck of pickled peppers Peter Piper picked
If Peter Piper picked a peck of pickled peppers
Where's the peck of pickled peppers Peter Piper picked