Forget Peter Piper and his Peck of Pickled Peppers - psychologists have come up with what may be the world's most frustrating tongue twister.
It may not make much sense, but the phrase "pad kid poured curd pulled cold" completely defeated volunteers taking part in a US speech study.
Asked to repeat the phrase 10 times at a fast lick, many of the participants clammed up and stopped talking altogether, according to lead researcher Dr Stefanie Shattuck-Hufnagel, from Massachusetts Institute of Technology (MIT) in Boston.
"If anyone can say this 10 times quickly, they get a prize," she said.
The tongue twister study, presented at the annual meeting of the Acoustical Society of America in San Francisco, was conducted to shed light on the brain's speech-planning processes.
"When things go wrong, that can tell you something about how the typical, error-free operation should go," said Dr Shattuck-Hufnagel.
Spoken too quickly, certain combinations of sounds appear to make people lose control of their mouths.
Often, one sound seems to replace another. For instance, "toy boat" becomes "toy boyt", and "top cop" becomes "cop cop".
But when the researchers recorded misspoken sounds and analysed them they found that the mistakes could be more subtle.
At least some of the time, tongue twister mix-ups were not one sound or another, but appeared to be something in between.
In the "top cop" example, sometimes the "t" and "c" seemed to arrive at almost the same time ("t'kop") and sometimes there was a delay between the two sounds with space for a vowel ("tah-kop").
The scientists studied two categories of tongue twister, simple lists of paired words, such as "top cop", and whole sentences.
They found that in the word list tongue twisters there was a preponderance of "t'kop" type errors. In contrast, sentences produced more "tah-kop" mistakes that included a short vowel after the initial consonant.
One possible clue to what is happening may be the regular rhythm of the word lists compared with the more irregular timing of the sentences, said Dr Shattuck-Hufnagel.
But there appeared to be some overlap in the processes used to produce both types of speech.
The MIT team, working with colleagues in Germany, has already collected data from the next stage of the research, which involved placing tiny transducers on volunteers' tongues to measure their articulation.