'It's the oldest and simplest joke of all: the pratfall. If in doubt, fall on your face.'
That's from The Radio Times (the oldest British television and radio listings magazine) 27/4 - 3/5/2013.
Well, it made me laugh.
I suppose doing a pratfall while falling on your face is just about possible: but it would involve pretzel-like contortions.
Pratfall is marked in my Collins dictionary as a USA and Canadian word meaning to fall on the buttocks. Nowadays it's quite commonly used and understood in Britain (though, obviously, not universally understood, see above).
Never mind, it's a lovely word, pratfall, and prat is nice, too. In Britain it usually means an idiot, a fool, or an incompetent, but the old meaning of buttock still lingers in the insult, even if the word prat is no longer used in strictly anatomical senses.
The best thing about the word prat is that it's a reasonably polite way of insulting your friends (though not, if they're listening, those in authority), and as such will always be in demand and is greatly to be cherished.
Word To Use Today: prat. This word first appeared in English in the 1500s, when it meant buttocks. No one knows where it came from before that.