Use the mouse to drag the block all the way to the left and then click on the Action button to release it. You will notice that as soon as the Action button is clicked, the spring begins to do work on the block, accelerating it and giving it kinetic energy. Evidently between the time you left the spring compressed and the time you released the block with the Action button, the spring had the ability to do some work. The ability to do work, we said, is energy. This energy contained in the compressed spring is apparently of a different kind than the energy that the block has by virtue of its motion. It is stored energy, subject to use at a later time. Energy of this sort is called "potential" energy, pe. The pe stored in the spring at any position, x, is exactly the amount of work done on the spring in reaching that position.