“I would swing so high that the back legs would come off the ground, then the front legs would come off the ground.”
Years later, as he looked back on it, he realized what was happening - gravity acting on his weight made the swing set walk.
He mused on that thought and ended up with his invention, the Murphy Mover.
Clever. Putting blocks under the lifted legs must be an exciting job...
It's all in the timing.
;-)