Answer:
How about a spring?
There are two kinds of springs, and you can tell the difference by looking at them when they're not under load. We'll call one a pulling spring, and the other a pushing spring. A pulling spring has all the coils touching when it's just laying there; a pushing spring has the coils spread out. I'm calling them what I'm calling them because you load a pulling spring by pulling on it and it pulls to return to its at-rest state, and you load a pushing spring by pushing on it (or "compressing" it) and it pushes to return to rest.
If you load a pushing spring, it will contain potential energy equivalent to the load you put on it. Remove the load, and the spring will relax to its at-rest state. If the spring's big enough and you allow it to relax in an uncontrolled fashion, it can do terrible damage--one of the warnings we give mechanics who work on suspension springs is, "if you don't secure the spring while you're releasing the tension, it will take your head off." And it will.