Here's an interesting subtlety of modern door handles. Next time you're in a high-rise building, check the door handles on the stairwells. You may find that the

Here's an interesting subtlety of modern door handles. Next time you're in a high-rise building, check the door handles on the stairwells. You may find that the movement of the handle on one side of the door is mechanically uncorrelated with the movement of the door handle on the other side; yet other, identical-looking door handles in the same building may function normally. I believe this is a safety feature to prevent doors from being jammed on the opposite side by putting e.g. a piece of furniture underneath the door handle. Someone had to actually think of this risk, and invent this special type of door handle - yet most people will never notice it.