Stolen from a couple of websites.
Compared to the rotations that one usually sees (tires on a travelling automobile, a compact disc playing music, or a draining sink), the rotation of the Earth is very small: only one rotation per day. The water in a sink might make a rotation in a few seconds and so have a rotation rate ten thousand times higher than that of the Earth. It should not be surprising, therefore, to learn that the Coriolis force is orders of magnitude smaller than any of the forces involved in these everyday spinning things. The Coriolis force is so small, that it plays no role in determining the direction of rotation of a draining sink anymore than it does the direction of a spinning CD.
The direction of rotation of a draining sink is determined by the way it was filled, or by vortices introduced while washing. The magnitude of these rotations may be small, but they are nevertheless gargantuan by comparison to the rotation of the Earth. I decided to include a picture of a draining sink, and the first one I tried in my house was found to drain clockwise (the opposite of what the silly assertions would have it do here in the northern hemisphere). This direction was determined entirely by the way the tap filled the sink. The direction of rotation of a draining toilet is determined by the way the water just under the rim is squirted into the bowl when it is flushed.
A surprisingly large number of my undergraduate students tell me that their high-school teachers told them that sinks drain in opposite directions in the two hemispheres owing to the rotation of the Earth. Why would a teacher offer such garbage to students when it is so easy to check. A trip to the school washroom (let alone the ones at home) will reveal drainage in both directions (which would certainly require the equator to assume a tortuous track through the countryside).
Is knowledge just a bunch of abstractions to be memorized with no recourse to the relevance of everyday experience?
Sigh... I dont know why teachers do this. I can but assume that those who do so just never feel any need to wash their hands --- or their minds.