1. The Water in the Bowl Is a Lid Made of Water
Under every toilet, a pipe leads away to a sewer or a septic tank. That pipe holds smelly air, and in a sewer it can also hold gases and small pests. Without something to block the way, that air could drift straight back up into the room.
The fix is a bend in the pipe that always holds some water. Plumbers call the bend a trap, and the water sitting in it a water seal or trap seal. (In Japan, plumbing manuals call it fūsui, which means "water seal" too.) Air has to push through water to pass, and it mostly will not. Sinks, tubs and showers all have traps; for a toilet, the trap is built into the bowl itself, so the water you see is the seal [1].
So the water in the bowl is the whole trick. It stays because the pipe bends up before it goes down, and a bend that rises again cannot drain completely. When you flush, new water pushes the old water over the high point and then refills the dip.
2. A Clockmaker Patented the Bend in 1775, and It Did Not Catch On Right Away
In 1775, a Scottish watchmaker named Alexander Cumming received the first British patent for a flushing toilet. The heart of his design was an S-shaped bend in the pipe, which is what we now call an S-bend or U-bend. His patent described "the stink trap so constructed that its contents shall or may be totally emptied every time the closet is used" [2]. In other words, every use replaced the water in the trap with fresh water.
Three years later, in 1778, the inventor Joseph Bramah patented a different way to do it. His patent described two valves worked by one handle, with one valve under the bottom of the bowl that, when closed, "retains any water that may be therein, thus cutting off all smell" [2]. Bramah added a hinged valve to Cumming's idea, and the Science Museum in London says this became one of the leading designs for the next hundred years [3].
Both inventors were after the same thing: a bit of water standing in the way of the smell. But the toilets could not spread yet. The National Archives notes that houses were not equipped with indoor plumbing or running water, and the flushed waste had no sewers to flow into [2]. London's real turning point came in 1858, the summer of the "Great Stink," when the smell from the River Thames got so bad that Parliament closed. The government started a huge sewer-building project that same year [3]. As sewers were built, keeping smells from coming back up the pipes became a bigger concern.
Then, in 1852, the plumber Josiah George Jennings described a toilet with the pan and the trap made as one piece, so that "there shall always be a certain quantity of water retained in the pan itself, in addition to that in the trap" [2]. The National Archives says his design is still recognisable as the basis of most modern flush toilets [2]. Keeping water standing in the bowl is a feature we still rely on, more than 170 years later.
3. Why Plumbing Codes Say the Water Seal Should Be 2 to 4 Inches Deep
How deep should the seal be? The answer is written down in plumbing codes. The Oregon Plumbing Specialty Code (2011 edition, which follows the widely used Uniform Plumbing Code) says each fixture trap "shall have a liquid seal of not less than two (2) inches (51 mm) and not more than four (4) inches (102 mm)," unless the local authority decides a deeper seal is needed [4]. A drainage manual from the city of Shiroi in Japan sets a similar range, 5 to 10 cm (roughly 2 to 4 inches), for the drain-pit traps it covers [5]. Two very different places ended up with almost the same numbers.
Why a minimum? A seal that is too shallow can be emptied by small losses, and then the lid is gone. Why a maximum? The same Oregon chapter says every trap, apart from special ones inside grease interceptors, must be self-cleaning, with a smooth, uniform inside surface [4]. A simple way to picture it is that a very deep pool of water is harder for the flowing water to stir and rinse away, so dirt could settle in it. That is our way of picturing the rule, not an explanation given by the code.
One more thing to know: these numbers are code limits for traps in general. The water you see in a particular toilet bowl depends on how that model is built, so the depth is not the same in every bowl.
4. How a Water Seal Can Disappear, and How Plumbers Protect It
A seal is only a lid while it is there. Two things can take it away.
Water in the trap can evaporate. A toilet gets refilled every time it is flushed, but a rarely used drain, such as a floor drain in a basement, can slowly dry out. The code deals with this directly: floor drains connected to the drainage system must have "an approved automatic means of maintaining their water seals" [4]. A device that tops up a trap with a little water now and then is called a trap primer [4].
Moving air in the pipes can pull or push the water out. The code requires each fixture trap to be "protected against siphonage, back-pressure" by a vent pipe, which lets air in so that draining water does not suck the seal out [4]. This is also why the code prohibits "S" traps, bell traps and traps with no vent [4]. The pipe shape that Cumming patented is now used together with a vent, which the code requires to protect the seal.
So the lid made of water depends on two quiet helpers: a bend that holds the water, and a vent that keeps the water from being pulled away.
5. A Safe Way to Watch Water Disappear by Evaporation
You can see why a seal in an unused drain might shrink with a plain cup. Fill a clear cup with water, and put a strip of tape at the water level. Leave it for three days, then compare the water to the tape. For a fair comparison, put a second cup with a lid beside it. Choose a warm, dry place where the cups cannot be knocked over, and ask an adult before moving them near anything that can be damaged by water.
This is only a rough look at evaporation. It does not tell you how fast a real trap loses water, because that depends on the room and the pipes. If you want to read more, look up your local plumbing code, or a city or water utility's page on "trap seal" or "floor drain." The sources below are a good place to start.
Next time you flush, watch the bowl refill and look at the surface for a moment. That small pool is a piece of engineering that has been quietly doing its job for about two and a half centuries.