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
I am not a pheasant plucker, I'm a pheasant plucker's son but I'll be plucking pheasants When the pheasant plucker's gone.