Carol is traveling by ship from south of the equator to the north. She has a nice little second class cabin which has a bathroom, but no window. Carol has no compass nor other instruments. Yet, without leaving her room or talking with anyone, Carol knew exactly when the ship crossed the equator.
How did she know?
When she thought she was nearing the equator, Carol filled the sink and allowed the water to drain away down the plughole. When the water reversed direction when going down the drain, she knew the ship had crossed the equator.
Why this works
This riddle plays on the concept of the Coriolis effect, which is the phenomenon that causes fluids to rotate as they drain due to the Earth's rotation. In the Northern Hemisphere, water typically drains clockwise, while in the Southern Hemisphere, it drains counterclockwise. By observing the direction of the water draining from the sink, Carol was able to deduce that she had crossed the equator, as the change in drainage direction indicated she had moved from one hemisphere to the other. It's a clever twist that highlights a scientific principle in a fun and engaging way!