A tongue twister is an alliterative group of words designed to twist the tongue and to help students and children to practice different sounds. Especially with languages like English and French, tongue twins assist the students in their mastery of the pronunciation of difficult letter combinations.
Tongue twisters help students with their difficulties in pronouncing some vowel and consonant combinations. Portuguese students especially have a problem with pronouncing combinations like 'th', 'ea', and 'sh'. Dr. Seuss' book Fox in Socks contains some fun examples for more advanced English students. Shorter, more common tongue twins like 'She sells seashells' are another good choice for a student who needs practice with these more difficult sounds as they train the student's ear as well as their tongue. I would also point out that a twister like 'unique New York' — when it is said over and over again — is a stumbling block even for native English speakers.
Five minutes spent reviewing the recently learned tongue twins and another five minutes of careful practice on a new one is plenty of time to spend on this exercise during a lesson and assures that the students already know the few rhymes (the teacher should say the twister slowly and encourage the student to say it out loud until he is able to say all the words with the correctly). The student should practice saying the it aloud as part of his homework, focusing especially on the saying the sounds clearly. When he is comfortable saying the rhyme slowly, the student should gradually aim for increased speed, but never at the expense of clarity.
Children naturally adore playing with words, and many students find this aspect of the lesson one of the most entertaining and stimulating parts, but even older students find it challenging and useful. If you have more than one student you can make a game out of increasing the speed without sacrificing clarity, and turn it into a friendly competition to see who can say it fastest without stumbling. The tongue twins themselves can also be used to illustrate certain points of grammar or to introduce new vocabulary although that is not the main point of the exercise. If this game is reserved for the end of the lesson, it gives the students an incentive to work hard through the lesson and some stimulating homework.
Tongue twins are a fun tool which can be useful in the serious work of acquitting correct pronunciation. If you have spoken to many non-English speakers you will know that sometimes an inability to pronounce these common sounds leads to embarrassing objections of meaning. Just as we want to pronounce a foreign language correctly, so we want to help our students gain an English pronunciation which helps them communicate.