In Part 3 of The Tree House, Sid and his classmates have Rug Time with Miss Susie and discuss how to lift heavy things to high places. Miss Susie tells the students about a simple machine that uses a wheel attached to a hook and has a rope running through it to hold onto the object: a pulley. The students learn that pulleys help lift things up and pull things sideways.
In Part 4 of The Tree House, Sid and his classmates reflect on their task of getting the bucket of tennis balls up onto the playground. Sid realizes that he can apply what he has learned about pulleys to his problem of the toys and the tree house.