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 U.S. 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.