A tongue-twister is a phrase that is designed to be difficult to speak properly.
Usually tongue-twisters rely on similar but distinct phonemes (e.g. s and sh), unfamiliar constructs, or other features of a language.
Tongue Twisters, amazing fun with friends and family
Tongue Twister of the Day
A woman to her son did utter, "Go my son, and shut the shutter." "The shutter's shut," the son did utter, "I cannot shut it any shutter.