“Water drains in opposite directions in the Northern and Southern hemispheres”

The idea that water drains in opposite directions in the Northern and Southern hemispheres, often attributed to the Coriolis effect, is a widespread misconception. While the Coriolis effect is a very real phenomenon, influencing massive systems like ocean currents and the rotation of hurricanes and large weather systems, causing them to spin differently depending on the hemisphere, its influence is drastically overstated when applied to household plumbing.

The scientific truth is that the Coriolis force is far too weak to have any discernible impact on the relatively small volume of water and short drainage time of a sink, bathtub, or toilet. Instead, the direction water swirls down a drain is determined by much more immediate and localized factors. These include the geometry and imperfections of the basin, any residual motion already present in the water from filling or disturbance, and even the design of the drain itself. You can easily make water drain clockwise or counter-clockwise in the same sink by simply stirring it gently before pulling the plug.

This engaging myth likely persists because it connects an everyday observation to a grand scientific principle, making it intuitively appealing. It has also been perpetuated by popular culture and, notoriously, by hucksters at the equator who perform rigged demonstrations for tourists, subtly manipulating the water to create the desired effect. While it's theoretically possible to observe the Coriolis effect on water in a draining basin, it requires an almost perfectly still, perfectly symmetrical container, with water left undisturbed for days and draining through a tiny hole over many hours—conditions far removed from any typical household drain.