There are many examples of simple, innovative designs, from the paperclip to the pull tab. But look under your sink for a brilliant design unchanged since 1775.
With zero moving parts, it uses the very water it drains to block explosive sewer gases from entering your home. It is the plumbing trap.
Before the late 18th century, indoor plumbing was just a straight pipe connecting a sink or early toilet to a cesspit below. The problem with a straight pipe is that it acts as a two-way street. While wastewater goes down, fumes—methane, hydrogen sulfide, ammonia, and the putrid smell of human waste—come right back up. These volatile gases made early indoor plumbing almost unbearable and frequently dangerous.
In 1775, a Scottish watchmaker named Alexander Cumming realized he could solve the problem just by changing the geometry of the pipe. He bent the pipe into a sharp "S" curve (which was later refined into the wall-mounted "P" trap used under modern sinks).
A modern P-trap under a sink. The dip in the pipe retains enough water to block sewer gases from traveling backward. - Photo by McGeddon (Wikimedia Commons) is licensed under CC BY-SA 3.0
By forcing the pipe to dip downward and then immediately curve back up before continuing to the sewer, Cumming ensured that a small amount of water would always pool at the bottom of the curve after the water was shut off.
That trapped pool of water acts as an airtight, liquid seal. When you run the faucet, gravity pushes new water through the trap, washing the old water out. But the moment the faucet is turned off, the last bit of water settles in the bottom of the dip. This small physical plug blocks foul odors, toxic fumes, and insects from traveling back up the pipe and into the home.
Without the simple curve of the plumbing trap, dense modern cities could not exist. The invention allowed toilets and sinks to be safely integrated directly into the living quarters of homes, apartment buildings, and skyscrapers without turning them into open exhaust vents for the municipal sewer system.