Crossing The Equator

Q: Sally 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, Sally will be able to tell when the ship has crossed the equator. How?

A: Sally 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.