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.
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