If you pour a little boiling water into an empty plastic jar (instant coffee jars work great), then pour it out and close the lid, and wait a few seconds for it to cool, the jar will collapse in on itself. When you poured the boiling water in it, the air inside the jar got very hot and expanded, but pouring the water out prevents it from remaining hot, so it will cool quickly and contract, so the jar will collapse in on itself, especially at weak points like the large areas around the side as opposed to the stronger points like the neck.
