Scientists from the Massachusetts Institute of Technology (MIT) have come up with a string of words so difficult that test subjects couldn't even finish it.
"Pad kid poured curd pulled cod" stumped participants in a recent study so badly that some of them simply stopped talking altogether when they tried to say it.
"If anyone can say this 10 times quickly, they get a prize," said MIT psychologist Stefanie Shattuck-Hufnagel when she presented her team's work at a meeting of the Acoustical Society of America.
Fortunately, the phrase — which was deliberately crafted to induce a kind of speech mistake that linguists call "double onsets" — is unlikely to come up in everyday conversation.
Dr Shattuck-Hufnagel and her colleagues didn't invent the tongue twister just to laugh at people trying to say it.
Mistakes caused by tricky phrases offer insights into how the brain processes speech.
"When things go wrong, that can tell you something about how the typical, error-free operation should go," Dr Shattuck-Hufnagel said.