<P> The focus of this article is a flush toilet that uses a mechanical flush from a water cistern that is above the toilet . </P> <P> A typical flush toilet is a vitreous, ceramic bowl containing water, plus plumbing to rapidly fill it with more water . The water in the toilet bowl is connected to a hollow drain pipe shaped like an upside - down U connecting the drain . One side of the U channel is arranged as a hollow siphon tube longer than the water in the bowl is high . The siphon tube connects to the drain . The top of the upside - down U-shaped drain pipe limits the height of the water in the bowl before it flows down the drain . If water is poured slowly into the bowl it simply flows over the rim of the upside - down U and pours slowly down the drain--thus the toilet does not flush . The standing water in the bowl acts as a barrier to sewer gas coming out of the sewer through the drain, and also as a receptacle for waste . Sewer gas is vented through a separate vent pipe attached to the sewer line . </P> <P> When a user flushes a toilet, a "toilet flapper valve" (not to be confused with a type of check valve) opens and allows water from a reservoir tank to quickly enter the toilet bowl . This rapid influx from the tank causes the swirling water in the bowl to rapidly rise and fill the U-shaped inverted siphon tube mounted in the back of the toilet . This full siphon tube starts the toilet's siphonic action . The siphon action quickly (4--7 seconds) "pulls" nearly all of the water and waste in the bowl and the on - rushing tank water down the drain--it flushes . When most of the water has drained out of the bowl, the continuous column of water up and over the bottom of the upside - down U-shaped drain pipe (the siphon) is broken when air enters the siphon tube . The toilet then gives its characteristic gurgle as the siphonic action ceases and no more water flows out of the toilet . After flushing, the flapper valve in the water tank closes; water lines and valves connected to the water supply refill the toilet tank and bowl . Then the toilet is again ready for use . </P> <P> At the top of the toilet bowl is a rim with many angled drain holes that are fed from the tank, which fill, rinse, and induce swirling in the bowl when it is flushed . Mounted above the toilet is a large holding tank with approximately 1.2 to 1.6 U.S. gallons (4.5 to 6.1 L) of water in modern designs . This tank is built with a large drain 2.0 to 3.0 inches (5.08 to 7.62 cm) diameter hole at its bottom covered by a flapper valve that allows the water to rapidly leave the holding tank . The plumbing is built to allow entry of the tank's water into the toilet in a very short period . This water pours through the holes in the rim and a siphon jet hole about 1.0 inch (2.54 cm) diameter in the bottom of the toilet . Some designs use a large hole in the front of the rim to allow faster filling of the bowl . </P>

Where does the water from the toilet come from