The way that the water in your toilet swirls is determined by the way
that your toilet is built, not which hemisphere you live in. The toilet
shoots the water into the bowl at an angle, which determines which way
the water swirls.
What you may have been thinking of, though, is the way that water
swirls when it drains out of your sink or bathtub. Old science
textbooks did say that this is different in the Northern vs. Southern
hemispheres. Unfortunately, this is one of the many cases where you
can't always believe what you read.
The reason that the old books gave is the Coriolis effect, which
has to do with the way that the Earth rotates out from under something
that moves freely. (You can picture this by imagining a cannon that
sits on the equator and fires straight North. As the planet rotates out
from under it, the cannonball will appear to veer to the right, or go
clockwise. If the same cannon were to be fired straight South, the ball
would still appear to veer to the right, but it would be moving
counterclockwise with respect to the ground.)
However, this effect isn't strong enough on water to actually
determine which way the water will swirl. (It's a nice theory, though.)
There are far too many other things that can effect it, like the slight
pre-existing movements of the water.
(republished on 07/31/06)