In 1775 Alexander Cumming, a Scottish watchmaker, patented a flushing toilet, and the part of it that mattered most did not move at all. He bent the waste pipe into an S. Water settles in the low curve of the bend and stays there, and that small pool is the whole wall between the house and the sewer: the smell cannot climb through it. Each flush sweeps the bowl clear and leaves fresh water standing in the bend for next time, so the machine’s most important work is done by the little it fails to get rid of. A man who spent his working life on moving parts is remembered for a thing that works by staying still.