Sue is traveling by ship from south of the equator to the north.
She has a nice little cabin with a bathroom, but no window.
Sue 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, Sue will be able to
tell when the ship has crossed the equator. How?
Sue 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.