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 mother 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!"