A tongue-twister is a sequence of words that is difficult to pronounce quickly and correctly. Even native English speakers find tongue-twisters difficult to say quickly. Try them yourself. Try to say them as fast as possible, but correctly!
Feel free to add a tongue-twister of your own.
She sells sea-shells on the sea-shore.
Three grey geese in green fields grazing.
How much wood would a wood-chuck chuck if a wood-chuck could chuck wood?
A wood chuck could chuck all the wood a wood chuck could chuck if a wood chuck could chuck wood.
Red leather, yellow leather, red leather, yellow leather.