When you press the flush lever, it lifts a flapper in the tank that lets the water in the tank flow into the bowl. The bowl flows into the p-trap underneath. When the p-trap overflows, it creates a siphon effect that sucks the remaining water from the bowl into the waste pipe.

The waste pipe connects to a grid-like sewer system that uses a series of sloped pipes and storm risers to carry the waste water to a treatment plant where it's filtered, disinfected, tested, and pumped back into the water system.

There are many way to deliver water, but the most common is the water tower. Water in the tower creates pressure that pushes water through the pipes. When you want water, you open a valve to relieve the pressure and water takes the path of least resistance.

When you flush a toilet, the valve floats. As the water drains from the tank, the valve lowers with the water level until it opens. As the tank fills, the valve raises with the water level until it closes. Once the valve closes, the flush cycle is complete and the toilet is ready to be flushed again.