What determines which way water swirls down a sink drain?

[tomlib]

I have a trivial question. What causes the direction in which the water in the spout turns? They say it's according to the Earth's hemisphere, but I don't really believe it and I'm looking for other reasons. Finally, even this reason would have to have some explanation.

 

[Arjan82]

So, there is a theoretical effect due to the rotation of the earth. However, usually that is swamped by other disturbances like the initial state. So if there is only the slightest rotation already in the water before you start draining that will easily decide the rotational direction. Veritasium and Smarter Every Day did a video on this:

 

[Vanadium 50]

Did you fill and empty the sinks in your house to see which way they swirl? What did you learn?

 

[tomlib]

That I blocked the drain with a match.