system
Given the following post, respond as if you're commenting on Reddit:
human
Why does water flow out in a twister when I drain the bathtub?
gpt
You can visualize this with a thought experiment.

Imagine that the water in the tub is not 100.00000% still before you open the drain.

Now sit atop a microscopic skin flake that is moving with this not-still water, in a random direction, not straight to the drain. Say you are moving to the right of the drain originally.

Now you open the drain and at first there is no twister. It just starts sucking water towards it.

You were moving to the right of the drain, but now the drain is sucking water, so there's a force pulling you towards the drain.

You already had a velocity, to another direction, but now you start accelerating towards the drain. You won't immediately stop going where you were going, make a sharp turn, and start moving straight to the drain (only cartoons behave like this). Instead, there will be a gradually increasing velocity component vector towards the drain, which is added to your original velocity vector which was to the right of the drain.

Your original velocity to the right is decreasing (unless you are rowing) naturally, and the velocity towards the drain is increasing. What's the result? You start curving towards the drain.

If you are imagining this right, you now see that you'll describe a gentle spiral towards the drain, right? And you can see that your speed will increase as you spiral to the center (like when you close your arms while spinning on an office chair).

But you're not alone in the bathtub. All "particles" of water will be subject to this sort of effect, and also interfere with other particles' movements (the magic of fluid dynamics). 

If the overall average random movements were more clockwise, they will add up and the resulting twister will be clockwise and vice-versa (forget the folklore about south/north of the equator; that effect is too minuscule to make a difference; you can force the direction by stirring the water around the drain by hand before or after opening the drain).

Even a small average direction tendency grows into a well defined twister, because once the rotation starts and it's accelerated ultimately fed by gravity, the rotating stream drags water around it to the same direction (positive feedback). 

You can also picture this imagining you're driving a car in a straight line and suddently you see your arch-enemy you must run over already at your left. You yank the steering wheel to the left, but you can't make a sharp 90 degree turn in real life; you describe an arc, and drift around the guy, spiraling towards him.