Here's why:

* Inclined plane: An inclined plane is a flat surface tilted at an angle. It's used to move objects up or down with less force than lifting straight up.

* Zipper mechanism: The teeth of a zipper act as tiny inclined planes. As you pull the slider, the teeth on one side of the zipper slide up the inclined plane of the teeth on the other side.

While zippers are not the most obvious example of inclined planes, the basic principle of reducing force by using a gradual slope is present in their design.