Why on Earth? Coriolis effect.
Is it true that water swirls down the toilet clockwise in the southern hemisphere and anti-clockwise north of the Equator?
No. It is true that in absolutely ideal conditions it will do this, thanks to something called the Coriolis Effect, which describes how the Earth’s rotation affects moving objects. And it’s true that very large objects - such as hurricanes - do observe this law. In practice, though, the Coriolis Effect is so weak that when only small amounts of water are involved it is easily over-ridden by all manner of other factors. If your loo is not absolutely level and perfectly moulded, for instance, the Coriolis Effect goes out of the window, so in reality water swirls away in both directions on either side of the Equator. If your toilet was about a mile wide then you could probably observe the effect in action. But then if you had a toilet a mile wide you’d have other problems on your mind.