  The purpose of this paper (which, in many passages, I take the liberty of
writing in the first person singular for bringing a personal experience) is to
show how teachers of Physics and Mathematics in Basic Education can teach,
through a simple and playful activity, the rules of signs that are related to
basic arithmetic operations involving positive and negative numbers. How? By
using a ruler, or any other object that can be interpreted as a number line,
and a flat mirror. By the way, as the justification of why this method works is
physical and mathematical, this paper provides a succinct review on the
formation of images by flat and spherical mirrors since some teachers of
Mathematics may (just as a possibility) not know how it happens. Lastly, given
the interdisciplinary nature of this activity, some additional considerations
are also presented regarding how these teachers can use it to instigate the
students to observe how Physics and Mathematics, together, are part of everyday
life for all of us.
