We use the context of astroparticle physics to introduce several fundamental concepts in physics and astrophysics. An activity has been developed using inexpensive materials that allows the reconstruction of the impact point and arrival direction of a cosmic ray particle measured by the Pierre Auger Observatory. The activity includes the discussion of fundamental concepts such as conservation of energy and momentum, centre of mass, trajectory, coordinate systems, speed, geo-orientation, time references and sky observation. (Contains 1 table, 8 figures, and 3 footnotes.)