Final answer:
Toilet water rotation is influenced by directional water jets more than by the Coriolis effect, which would only have a minor influence in the absence of these jets. The Coriolis force induces a slight counter-clockwise spin in the Northern Hemisphere.
Step-by-step explanation:
When a toilet is flushed or a sink is drained, the water and other materials start to rotate about the drain. The rotation in the Northern Hemisphere is often thought to be caused by the Coriolis effect, which is due to the rotation of the Earth. However, this is a very small effect and in most toilets, the rotation is actually caused by directional water jets designed in the toilet. If the water were pushed up instead of being drained out, the directionality of these jets would determine the rotation rather than the Coriolis effect.
In terms of the Coriolis effect, if there were no initial rotation and a flow directly straight toward the drain, the Coriolis force would cause a slight rotation to begin. The direction of this rotation would be counter-clockwise in the Northern Hemisphere.