Linguistics

You might be surprised to learn that this short phrase won the title for the most difficult tongue twister in the world: "Pad kid poured curd pulled cod."

Uh. Excuse me? Most of the world does not speak English. And there are plenty languages with tongue twisters that make English ones look like child's play. Example from German:

• Schnecken erschrecken, wenn sie an Schnecken schlecken, weil zum Schrecken vieler Schnecken Schnecken nicht schmecken.
• [ˈʃnɛkən ɛɐ̯ˈʃʁɛkən vɛn zi: an ˈʃnɛkən ˈʃlɛkən vaɪ̯l t͡sʊm ˈʃʁɛkən ˈfi:lɐ ˈʃnɛkən ˈʃnɛkən nɪçt ˈʃmɛkən]
• Snails get scared when they lick snails because, to the horror of many snails, snails do not taste good.

And "pad kid poured curd pulled cod" in doesn't sound specially difficult, even in contrast with other English tongue twisters. Typically a tongue twister will rely on one or more of the following:

• homosyllabic consonant clusters, specially alternating with non-clustering consonants. Like the German one above, note how it alternates [ʃ]+[ʁ n l Ø].
• two or more consonants with really similar points of articulation. Like English "she sells seashells by the seashore."; both [ʃ] and [s] are easy to pronounce individually, but since your tongue is going back and forth between both, you'll end saying stuff like "see shells sheasells" or similar.
• individual consonants with a tricky articulation.

For the later I got an example from Portuguese, that also shows language changes can affect those tongue twisters:

• o rato roeu a roupa do rei de Roma
• the rat/mouse chewed the king of Rome's clothes

For plenty people, this tongue twister isn't really difficult to pronounce, because they use fricatives for initial ⟨r⟩; stuff like [x ɣ χ ʁ h ɦ]. However, for people using older pronunciations, with either [r] or [ʀ] (both trills), suddenly this tongue twister becomes extremely hard.