Psychologists have come up with might be the world's toughest tongue twister, according to the Sydney Morning Herald. Ready? Here it is:

Pad kid poured curd pulled cold

It's tougher than it looks; at first I thought it would be easy because it is short, but it's not. According to Americanfolklore.net, the Guiness Book of World Records says this is the toughest:

The sixth sick sheik's sixth sheep's sick.

Try this one:

If you must cross a course cross cow across a crowded cow crossing, cross the cross coarse cow across the crowded cow crossing carefully.

And, finally, also from the American Folklore site, a poem:

I'm not the pheasant plucker, I'm the pheasant plucker's mate,
And I'm only plucking pheasants 'cause the pheasant plucker's late.
I'm not the pheasant plucker, I'm the pheasant plucker's son,
And I'm only plucking pheasants till the pheasant pluckers come