There are two glasses of wine, one white and one red. A teaspoonful of
wine is taken from the red and mixed in with the white. Then a teaspoonful
of this mixture is taken and mixed in with the red. Which is bigger, the
amount of red in the white or the amount of white in the red?
The answer is that the're both the same, because there's the same volume
in each glass, so whatever quantity of red is in the white must be equal
to the quantity of white in the red. However in practice it is impossible
to do this because the white always runs out first at parties and the red
always gets spilt on someone's white trousers.
-- R. Ainsley in Bluff your way in Maths, 1988