Soap turned the route into a sliding surface
Moving a lighthouse was not a matter of making it weightless. The tower still pressed down on its supports with enormous force; the challenge was getting those supports to travel without catching on the ground. Soap helped by reducing friction—the resistance that arises when surfaces rub against one another.
A thin, slippery layer between the supports and the prepared path made it easier for the tower to slide. Without lubrication, rough spots could snag or grind against the supports, wasting force and making motion less predictable. Soap did not carry the lighthouse or pull it forward: workers supplied the force, while the slick path reduced how much force was needed to overcome resistance.
Easier motion still required control
Lower friction makes an object easier to move, but it can also make its motion harder to stop or steer. A massive structure cannot be nudged casually once it starts sliding. The supports had to bear its weight, and the movement had to remain controlled so the tower did not shift off its intended route. The soap was useful only as part of that system, not as a substitute for sound supports or careful handling.
The practical lesson is that moving a heavy building is often about managing contact with the ground, not simply applying more muscle. In this case, an everyday cleaning product changed the behavior of the surface beneath the supports: instead of grinding forward, the lighthouse could slide.
