1. A Tap Is a Valve That Holds Pressurized Water Back
A tap, also called a faucet, is a valve that controls the release of a fluid. The word "tap" is the everyday one in the UK and much of the Commonwealth, while "faucet" is the most common word in the US [1]. (In this article, "tap" and "faucet" mean the same thing.)
Why does it matter that a tap is a valve? Water in a supply pipe is under pressure, so if the end of the pipe were simply open, the water would keep flowing out. That is a plain consequence of pressure, not a claim from one source. A tap therefore does three things at one outlet: it stops the water, it lets the water out, and it sets how much comes out. Stopping the water is one job; controlling how much comes out is another.
That gives us three questions to follow. How does the tap actually stop the water? How does it keep the water in your sink from ever sneaking back into the pipe? And what happens inside the pipe at the moment of stopping?
2. How an Old-Style Tap Stops Water With a Rubber Washer
In an older screw-down tap, turning the handle moves a shaft up or down. At the bottom of the shaft, a soft washer made of rubber or a similar material is pressed against a part called the valve seat. When the washer sits tightly on the seat, the water is blocked. When you turn the handle the other way, the washer lifts and water squeezes past. Wikipedia describes this design as a kind of globe valve, and notes that it seals well and lets you adjust the flow finely [1].
There is a catch. Pressing rubber against a seat again and again wears both parts and can corrode them, so after enough years the tap starts to drip [1]. The washer can be replaced and the seat can be resurfaced, but this kind of valve is never entirely maintenance-free [1]. A dripping tap is often not a mystery at all. It is the "holding back" job slowly failing at one small spot.
Not every tap works this way. Other designs, such as ball valves and single-lever mixer taps, use different parts to do the same stopping job [1], so the washer story fits older screw-down taps best.
3. Why the Spout Hangs Above the Sink: The Air Gap That Keeps Dirty Water Out
Look at a bathroom sink from the side. The end of the spout does not touch the basin. It sits above the highest level the water could reach before spilling over the rim. Plumbers call that empty space an air gap. In the model plumbing code used in much of the US, it is described as the vertical separation between the outlet of the drinking-water system and the flood level rim of the fixture [2].
The same code material calls an air gap the ultimate protection against backflow, because the best way to stop dirty water from flowing back into clean pipes is to have no connection between them at all [2]. The code's rule of thumb is that the gap should be about twice the width of the outlet opening, and never less than 1 inch (2.5 cm) [2].
A way to picture the danger is a drinking straw. If its end sits in a glass and you suck on the other end, liquid comes up. If pressure in the supply pipe dropped for some reason, and the spout were underwater, the pipe could in the same way draw water backward. That backward pull is called backsiphonage, which means water being sucked back into the pipe. The air gap stops the pipe from drawing water back from the sink. (This straw picture is a simple model. Real pipes are more complicated.)
The idea is not limited to sinks. Many places require a garden hose connection to have a vacuum breaker or other backflow preventer, which stops water in a hose from flowing back into the building or the public supply when pressure drops [1]. Faucets with a pull-out spray head solve it differently: the code says they may contain a built-in diverter that stops flow back if the spray head ends up under water, and such faucets are tested to a product standard [2]. So a tap can do more than hold water back. Some taps include extra parts that help stop sink water from flowing back into the supply.
4. Why Pipes Bang When You Shut a Tap Quickly: Water Hammer
Have you ever turned a tap off fast and heard a thump or rattle in the wall? That is called water hammer, or hydraulic shock. It is a pressure surge, or wave, that happens when a fluid in motion is forced to stop or change direction suddenly [3]. In homes it often sounds like a loud banging, and it is commonly linked to valves that shut abruptly, such as those in dishwashers and washing machines [3].
The water in a pipe has momentum, like a crowd of people walking in a line. If the front stops all at once, everyone behind pushes forward. That is only a picture, because water is not a row of separate people, but it shows why the pressure goes up near the closed valve and travels back through the pipe.
How big the surge gets depends largely on how long the valve takes to close [3]. Slower closing means a smaller pressure spike, which is why fitting slowly closing valves is one way to reduce water hammer [3]. Plumbers also add devices called water hammer arrestors, which act like a small cushion for the shock [3]. So the way you close a tap matters: stopping the water is not only a yes-or-no action, it is a change in how fast a column of water is moving.
5. How Much Water a Tap Lets Out in a Minute
A tap's third job is to control how much water comes out. In the US, the Environmental Protection Agency (EPA) says the standard flow for a bathroom sink faucet is 2.2 gallons per minute. Faucets that carry its WaterSense label use at most 1.5 gallons per minute, which the agency says cuts a sink's flow by 30 percent or more [4].
Let us turn that into something to picture. At 2.2 gallons a minute, a tap left running for one minute lets out about 8.3 liters, a little more than four 2-liter bottles. At 1.5 gallons a minute it is about 5.7 liters. (We did these conversions ourselves: 1 US gallon is about 3.785 liters.) Those are limits for new faucets. A real tap at home may give less or more, depending on its age, the water pressure, and how far you open the handle. That is why the next section asks you to measure your own.
6. Try It Yourself: Measure Your Tap and Look at the Gap
First, look at the spout of a sink from the side. Is there clear space between the end of the spout and the rim of the basin? Compare the taps on a bathroom sink and a kitchen sink, and an outdoor tap if you have one. Do you notice anything different about how each one is built?
Second, measure how much water comes out in one minute. Place a large measuring jug or a bucket in the sink, turn the tap on to a steady flow, and time one minute, then pour the water into 1-liter or 2-liter bottles and count. Try again with the handle only halfway open. Does half the handle mean half the water? Water is heavy, so ask an adult to help with full containers, and use the collected water for plants or cleaning rather than pouring it away.
Third, listen. Turn a tap off slowly, then a little faster (but do not slam it). Does the pipe sound different? If you ever hear a loud bang, tell an adult, because repeated hard shocks are not good for pipes [3].
If you want to dig deeper, the sources below are good places to start. The plumbing-code excerpt is a short, real example of how engineers write rules about keeping drinking water clean.