The right-hand rule for cross products was discovered by Foucault to calculate directions of electrical current, magnetic fields, and other such novelties. However, it is most famous for its ability to determine the proper orientation of a roll of toilet paper.
Contrary to Newton's Classical Theory on Lavatory (pronounced "lah-VOR-eh-tor-ee" by those dirty, dirty Brits) Paper which indicates a constant "over" or "under" orientation (quite frankly, no one seems to remember which), the correct orientation is given by pointing the thumb of one's right hand in the direction of the toilet from the dispenser. The outside sheets of the toilet paper should then curl around the tube in the same direction as the fingers.
Above is a toilet paper roll adopting, by the Newtonian classification, the "under" orientation. This setup would be correct if the toilet paper dispenser were to the left of the toilet.
The right-hand rule applies to 1- and 2-ply bathroom tissues; however, if 3-ply paper is in question, the left hand must be used, and that has a tendency to get more than a little messy.
This 1939 discovery ended centuries of debate on the subject, during which time a number of civilizations (Microsoft, France, and The University of Southern California, to name a few) felt themselves unable to use the bathroom whatsoever. Since that time, however, most other bathroom research has gone down the crapper.