Toilet, lavatory, loo, water closet, WC, John, crapper, can—it’s amazing we have so many names for something we care to talk about so little. Toilets are hardly the most glamorous of inventions, but imagine trying to live without them. About 40 percent of the world’s people (some 2.6 billion of us) are in that unhappy position, lacking even basic sanitation. At the opposite end of the scale, in Japan, people have amazing electronic toilets that do everything from opening and closing the lid automatically to playing music while you use them. Most of the world’s toilets are more modest than this, but they’re still pretty ingenious “machines.” Let’s take a closer look and learn how does a toilet work.

At first sight, toilets seem quite simple: you have a waste pipe going through the floor and a tank of water up above (called a cistern) waiting to flush into it when someone pushes a button or pulls a lever or a chain. Most flush toilets are purely mechanical: pull the chain and the cistern empties through the force of gravity, washing the bowl clean for use again. They are literally mechanical because they flush and refill using levers inside—and levers are examples of what scientists call simple machines.

There’s a little bit more to toilets than this. When you flush, the cistern has to refill automatically from a kind of faucet on the side and the refilling operation has to last just long enough to fill the tank without making it overflow. The “hole in the ground” is more sophisticated than it looks as well. You may have noticed that toilets always have a little water in the bottom of them; even when you flush them, they never empty completely. Some water is always trapped in a big curved pipe at the base of the toilet known as the S-bend (or S-trap). This little bit of water effectively seals off the sewage pipe beneath it, stopping germs and bad smells from coming up into your bathroom.

What happens when you flush?

  1. Press the handle to flush the toilet and you operate a lever (dotted line) inside the cistern.
  2. The lever opens a valve called the flapper (green) that allows the cistern to empty into the toilet bowl beneath.
  3. Water flows from the cistern through holes in the rim so it washes the bowl as well as flushing the contents away.
  4. There’s enough water flowing down from the cistern to flush the toilet around the S-bend (S-trap). Some water always remains at the bottom of the toilet, however, for hygiene reasons.
  5. The contents of the toilet are flushed down the main drain.
  6. As the cistern empties, the plastic float (red) falls downward, tilting the ballcock lever.
  7. The ballcock opens the inlet valve (green) at the base of the cistern, which works a bit like a faucet (tap). Water flows in, refilling the cistern, and pushing the float back up again.
  8. When the float reaches the correct level, the ballcock switches off the water supply and the toilet is ready to flush again.

(You can read the full article here) http://www.explainthatstuff.com/howtoiletswork.html