A popular misconception is that the Coriolis effect determines the direction in which bathtubs or toilets drain, and that water always drains in one direction in the Northern Hemisphere, and in the other direction in the Southern Hemisphere. This myth has perhaps been perpetuated by the Simpsons episode "Bart Vs. Australia," in which protagonist Bart Simpson has a young Australian boy confirm that his toilet water in the Southern Hemisphere spins in the opposite direction from Bart's own in the Northern Hemisphere. The myth is also propagated by websites claiming that it is true^[1]. Strangely, many of these sites claim that drain water spins clockwise north of the equator, and counterclockwise south of it, which is reversed from direction of spin that would result from the Coriolis force, if it were a determining factor.

The Coriolis effect is a few orders of magnitude smaller than various random influences on drain direction, such as the geometry of the sink, toilet, or tub, and the direction in which water was initially added to it. Most toilets flush in only one direction, because the toilet water flows into the bowl at an angle. If you shot water into the toilet basin from the opposite direction, the water would spin in the opposite direction^[2].

The Earth rotates once per day but a bathtub takes only minutes (and a toilet only seconds) to drain. When the water is being drawn towards the drain, the radius with which it is spinning around it decreases, so its rate of rotation increases from the low background level to a noticeable spin in order to conserve its angular momentum (the same effect as ice skaters bringing their arms in to cause them to spin faster). As shown by Ascher Shapiro in a 1961 educational video (Vorticity, Part 1) [1], this effect can indeed reveal the influence of the Coriolis force on drain direction, but only under carefully controlled laboratory conditions.