Here's a hypothetical that requires some analytical reasoning.
Which way do locks rotate: counter-clockwise, clockwise or both? Can you find determine the logic? Is there a preference for one direction or the other?
A clockwise motion to unlock a standard lock is most common, and is generally the preferable configuration. As Rixter noted, the vast majority (most estimates are ~90%) of the world is right handed and finds this turn more comfortable.
It is also more common to insert a key to unlock something than to lock it. Many residential and automotive locks will be locked by a button, twist or lever, which doesn't require a key to do. On automobiles, the driver's door will be the one unlocked most often - and at least in left-hand drive countries it better serves the driver to unlock the vehicle with a clockwise motion.
This is not a universal truth, however. Door locks are typically designed to unlock with rotation towards to opening edge of the door. This is something most humans have learned without even realizing it. But again, if all other factors are the same, the door should be designed for a right-hand dominant user.
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