"righty tighty, lefty loosy" Has anyone else heard that expression? It's supposed to be a way to remember which way to screw/unscrew things. It means that you turn the top of the screw to the right to tighten it, and you turn the top to the left to loosen it. What I'm wondering is, is this just an arbitrary convention that all screwmakers have decided on, or is there a physics principle behind it? I've never, ever, found something that screws in the other way, and it seems suspiciously like a vector cross product, which would seem to indicate that it's a physics principle. On the other hand, it also seems like all the threads in a screw simply go that direction, so I would think that one could reverse it simply by reversing the threads. Can anyone explain this to me?