I have one of those folding walking sticks--they're light and great when you travel. Something like this:
There is a strong rubber band holding the parts together--these are hollow aluminium pipes. All you have to do to fold the stick is to pull apart the parts and fold the thing.
So, anyway, I was entering the office one fine morning and the end of the stick got stuck when the front door closed behind me. The stick split apart into four parts, with the rubber band holding them together, so the stick stretched out and stopped me, and of course I stumbled. I caught on quickly and got up, and managed to push at the door behind me... and guess what rubber bands do when tightened and released?