How about: A skunk sat on a stump. The stump thought the skunk stunk, but the skunk thought the stump stunk.
Ugh, I remember one like that from Japanese class. The word translates as, “It was not hot,” or something like that. Romanized, it goes something like, “Attakakunakatta”. The teachers laughed at us a lot as we tried to stutter it out.
I don’t like the bottom parts of Pop Tarts, I like the top parts of Pop Tarts.
I think “sixth” is actually quite hard to say all on its own, so no wonder the “Sixth sick sheikh. . .” thing is hard!
Another one, like “toy boat,” that is easy enough to say once, but gets hard if you say it quickly and repeatedly, is “red leather, yellow leather.”
I wonder if the tongue twister phenomenon is related to the reversing of letters during hasty typing, producing things such as teh (I do this all the time, more often than I type it correctly, and have to have my word processor set to auto correct it.)