Contrary to popular belief, toilets do not drain in opposite directions in the Northern and Southern hemispheres. The direction in which toilets and bathtubs drain is purely determined by their design, and not, as it is often thought, by the Coriolis effect.
In physics, the Coriolis effect refers to the deflection of moving objects when viewed in a rotating reference frame. The magnitude and direction of the Coriolis acceleration is determined by a mathematical formula. The Coriolis effect is often discussed as it applies to motion taking place on the surface of the Earth as the Earth spins on its axis. It affects the direction of large-scale motions such as hurricanes, but its force is too weak to affect the direction of motion happening on a much smaller scale, such as a toilet draining.