The term “work” has a diverse meaning in general language use, but in physics we need to define the concept more precisely. We approach the physical concept of work through a simple machine, the tackle. We find an interesting relation, namely that the product of distance and force is constant. We demonstrate the relation experimentally and develop the concept of conservation of energy from it.Finally, we show that the concept of conservation of energy is already contained in Newton's axioms and we derive mathematical criteria that allow us to identify the situations in which the sum of mechanical energies is already conserved.