The modern toilet has been around awhile, but its workings remain a mystery to many people.
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The driving force that makes a toilet work is gravity. Water enters the toilet through a supply line equipped with a shutoff valve, or "closet stop," at the base of the toilet. After the water travels up the supply line through the base of the tank, it's diverted in two directions by an inlet-valve assembly sometimes called a ballcock. Most of the water goes into the tank, but some goes directly into the bowl via a fill tube that runs down through an overflow tube. As a result, as the tank fills up, so does the bowl.
How does the toilet know when to stop filling? As water fills the tank, a float ball rises until it reaches a certain point, where it begins putting pressure on the ballcock, which shuts off the flow of water. When you flush the toilet, the handle trips a lever that raises a lift rod or chain, which in turn raises a flapper that covers an opening at the bottom of the tank, called the flush-valve seat. When the flapper is raised, gravity pulls water from the tank into the hollow body of the bowl, then into the bowl itself through small passages just under the rim. The cyclone effect caused by water's rushing into the bowl is what carries waste down through the drain at the bottom of the bowl. Almost immediately after it opens, the flapper drops back into its seat, and the tank begins filling again.
Depending on the brand or style of the toilet, the parts inside the tank may vary, but they perform the same basic operations. For instance, the float device in older models is often made of copper, whereas newer ones are plastic. The mechanism lifting the flapper may be a rod or a chain. Flappers also come in different types. Older ballcock assemblies are sometimes made of brass; newer ones are plastic. Finally, some toilets don't use a float ball but instead have a float cup that slides up and down a tube as the water level rises and falls. This setup combines the float and ballcock into one unit. It achieves the same result as the float and lever but is a bit more precise.
Tip: The water in the toilet tank is clean because the tank is physically separated from the bowl. Some people put bleach in the tank in an attempt to remove rust stains or other discoloration from the porcelain sides. This practice should be avoided, as bleach and other chemicals can damage or shrink rubber seals and cause malfunctions.