I've always been fascinated by springs.
All sorts, in watches, screen doors, and the sort attached to garage doors. They help us tell time, automate our activities, or lessen the effort it takes to complete a task.
One of my earliest and favorite was a toy that came in a cardboard box, the Slinky. My first one was steel, with the ends attached to the main spring with a metal clip, so children didn't spear themselves on the sharp metal. Later, I had plastic varieties, often purchasing them for children's party favors, one to a child.