There was once a man who had 2 horses, but he couldn't tell them apart. So he cut the tail off one, but it didn't work, because it grew again. So he cut the mane off one, but it didn't work, because it grew again. Then he misured them, and he found that the black hourse was 4cm shorter than the white horse.
P.S. This "joke" was used (in an interesting teaching way) during my first lesson for the ESOL L1 course in the [en:Southwark College], just started today. P.P.S. I know: it's a shame to have a course in Saturday, at 9:30, when you are supposed to sleep. But, what can I do?