Whenever I wash the dishes, I realize that water always tend to acquire some kind of rotation with respect to the axis passing through the hole of drain. I'm not sure if my observations account for only one direction of rotation, but the rotation is certain every time. I think about a box with sand and a hole in the bottom, and I'm pretty certain that it should fall off in (almost) straight lines, so I want to know if this is a unique behaviour shown only in fluids, and why it is that way.
it is because of the conservation of angular momentum.
The water in the drain is always spinning very slowly, the only way it wouldnt rotate if it is small enough to not be affected by the coriolis effect of earth and if nothing disturbs the water. What you see when it is draining is that the linear rotation speed becomes evident as the water concentrates in a small area, because the revolutions per minute increase