The determination of some magnitudes in physics requires the use of the cross product. How to find the direction of the resulting vector is generally explained in textbooks using the right-hand rule.1,2 Teachers are aware of the difficulties students have with three-dimensional vector relationships, and different tools have been proposed to help them.3 A very simple device is presented here after one of my students’ suggestions (a 17-year-old girl named Priscila), a lipstick applicator.

The lipstick rule (or Priscila’s rule) for the cross product states: A lipstick applicator is placed on the plane containing the vectors whose product is being calculated, a × b ; then, the base of the tube is rotated, from vector a to vector b through a small angle, to get the lipstick out of the tube. The tip of the lipstick points in the direction of the resulting vector c (see Fig. 1...

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