The drain doesn't spin from Earth's rotation

It's one of those facts everyone repeats: in the northern hemisphere the sink water spins one way and in the south the other, because of the Coriolis effect, the same one that curves hurricanes. It sounds neat. And for your sink, it's false.

The Coriolis effect is real, but it's ridiculously weak at this scale. In a sink it's completely overwhelmed by much bigger things: the shape of the basin, how the water came in, the eddies left over from the last time you turned on the tap. That's why the same sink can spin one way today and the other tomorrow.

Coriolis does rule the huge and the slow: storm systems hundreds of kilometres across, ocean currents. Your drain is far too small to notice it lives on a spinning planet. Next time, empty the sink twice and watch for yourself.