An old chestnut
goes like this:
There are two gallon containers, one with a gallon of water, the other
with a gallon of wine. One teaspoon of the water is dumped into the wine,
and mixed thoroughly. Then one teaspoon of the resulting mixture is
dumped into the water, and mixed thoroughly.
Is there more water in the wine or more wine in the water? Ignore the fact
that wine contains water, and assume no change in volume due to mixing
(not true, but perhaps negligible for such a small amount of mixing)
and assume perfect mixing -- that is, the teaspoon of mixture contains
the same proportions of wine and water as remain in the "wine" container.