The static equilibrium properties of a spool, resting on an incline and subject to the tension exerted by a string wrapped around the core cylinder, are studied by means of Newtonian mechanics. The overall behaviour of this system is imagined to be similar to that of a doggie kept on a leash. Starting from the well-known mechanical properties of the spool on a horizontal plane, this analogy is used to construct an inquiry-based-learning lecture adopting the BSCS 5E instructional model. In this approach the static properties of the spool on the incline are used to conceive an Extend phase.