“Toilets flush in opposite directions in the Northern and Southern Hemispheres.”

The idea that toilets flush in opposite directions in the Northern and Southern Hemispheres is a persistent and widespread misconception. This myth likely gained traction from the 1960s, possibly after scientists at MIT demonstrated a very subtle Coriolis effect on water in a meticulously controlled bathtub experiment. However, this experiment required near-perfect, undisturbed conditions that are entirely absent in an everyday toilet. Pop culture, including popular television shows, further cemented this intriguing but incorrect notion in the public consciousness, making it a common talking point and even the basis for tourist "demonstrations" near the equator.

The Coriolis effect is a real phenomenon caused by the Earth's rotation, and it undeniably influences large-scale systems such as ocean currents and massive weather patterns like hurricanes and cyclones, causing them to rotate in opposing directions depending on the hemisphere. However, its influence is incredibly weak and only becomes noticeable over vast distances and extended periods. For small bodies of water, like those in a toilet bowl or even a bathtub, the Coriolis force is minuscule and completely overwhelmed by other, much stronger factors.

The direction water swirls down a drain is primarily determined by the design of the basin itself, including the angle of the water jets and the shape of the bowl, as well as any initial momentum the water has from being introduced into the toilet. Even tiny disturbances, such as a slight unevenness in the bowl or how the water initially enters, will have a far greater impact on the swirl direction than the Earth's rotation. Therefore, while the Coriolis effect is a fascinating scientific principle, its scale of influence does not extend to your bathroom plumbing.