Why does water circle down the drain in opposite directions in the Southern and Northern hemispheres.
It doesn’t. Here’s the straight dope.
I sould mention that the Coriolis effect does exist, but is so slight at small scales that other influences can (and often do) override it. So water does not always spin in opposite directions.
On an unrelated note:
Why does my cat like toilet water better than any other kind of water? That’s gross.
Laurasia I read somewhere (might have been Straight Dope) that toilet water can be cleaner then other kinds of water. Not sure if it had something to do with filtration, the amount of time we spend cleaning the bathroom vs. the kitchen, or whatever. I do remember this article saying something like “if an alien came to your house and didn’t know better, he’d probably drink from your toilet and shit in your kitchen sink”.
Or maybe it’s just colder water. Or perhaps your cat does it to piss you off. 
Ahhh, found it. It was a SD article. Link
I was in Rio Bamba, Equador, not long ago and came across a native resting under an olive tree. Since we were on the equator and I had a shallow knowledge of CORIOLIS, I asked my new acquaintance in my best Spanish, “Senior, do you know if the effect of coriolis is absent here in the equator?”
He cleared his throught and replied," Of course, we all know all about Coroplis in Rio Bamba. This pseudo force is the vectorial cross product of the rotational velocity of the earth crossed with the velocity of a moving body. Since these two vectore are in a horizontel plane for a body moving horizontally, the cross product is either up or down. If a body is moving due west in this locality the coriolis vector points directly down. I can prove it. Do you see that dog trotting across the street there. He is going west so when he decides to stop and rest under that tree, he will neither wind to the right nor the left, he will simply crash straight on. The mutt did exactly that. QED
Hell of a first post, hc_paul! Welcome to the Dope!
It’s colder. It’s sitting in that nice big porcelain bowl, which keeps it chillier than that dumb old plastic bowl in the kitchen.
And he’s doing it to piss you off. Cats are like that, ya know.