If one were to take a crushable plastic bottle and crush it, and, right after crushing, keep the bottle cap , the bottle would retain it's crushed state. However, if the bottle cap were not to be kept, then the bottle would slightly recover in shape.
Why does keeping the cap on reduce the ability to reform?
When you leave the cap on, no air can flow into the bottle, so you can't do deformations that change the volume inside the bottle as easily anymore since then the pressure inside the bottle would change compared to the pressure outside and then there's a net force on the walls of the bottle due to the pressure difference.
