A simple spring, like a slinky, has potential energy when it is compressed and when it is stretched. When it changes or is released from either state, it is capable of doing work. This energy, called elastic potential energy, is energy that is stored in anything flexible. It is contained in materials that can stretch when forced but can later return to their original shape.
The picture above shows a spring being stretched by someone. What do you think is going to happen when the person lets go of the spring?
A few common examples are a sling shot, spandex or a rubber band. Imagine pulling back on a sling shot.
You are actually creating elastic potential energy, EPE. When you release the band, it is capable of doing work. It can accelerate a ball across a field!