One of the shortest and, according to the Guinness Book of World Records, the most difficult tongue twister in the English language. (You have to accomplish it five times fast in order to claim it's not.)
It's not the hardest tongue twister in any language, however. If you include all of Europe, that prize goes to the Czech sentence "Strch prst skrz krk", a vowel-less wonder which translates as "stick a finger down my throat" -- which is pretty much what it sounds like is happening when you try to pronounce it.
If you go completely global, however, the winner is in Xhosa: "Iqaqa laziqikaqika kwaze kwaqhawaka uqhoqhoqha", which means "the skunk rolled down and ruptured its larynx."