Susy is traveling by ship from south of the equator to the north. She has a nice little cabin with a bathroom, but no window. Sally has no compass nor other instruments. Just the general luggage one brings on board a long cruise. Yet, without leaving her room or talking with anyone, Susy will be able to tell when the ship has crossed the equator. How?
Susy can fill the sink and watch it drain. When the water reverses direction when going down the drain, she will know they have crossed the equator.