system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does draining a large body of water cause it to form a vortex?
gpt
The vortex that forms is caused by the physical principle called angular momentum. Let us assume a straight walled circular pond, with a drain at the center bottom. As the water begins to drain, the flow of water comes from the edges towards the center. This creates momentum at an angle from the direction the water is actually being allowed to move - straight down. The further (and thus faster) the water travels on the way towards the drain point, the more angular momentum it has.

You can envision this as a golf ball just barely hitting the hole - it skirts around the edge of the cup before dropping in (Or if you're really unlucky, skirts the edge and changes direction before not going in). More or less the same is happening to the water. It has speed in a direction that isn't straight down the drain.

This is what forms the vortex. A common misconception is that the direction (and the cause of the vortex) is the Coriolis effect, which is caused by the earth's rotation. This is actually not true. Although you can create conditions where you can observe the coriolis force creating vortexes that always go in the same direction (Depending on your hemisphere), most times the direction of the vortex is random, if the shape of the vessel or the drain doesn't affect the direction the vortex would form.