Explain how a three-way light switch works (electrical engeering)

please and thanks

* A paragraph that describes the operation of the circuit. For example, imagine you were speaking to someone who is as “far” from being an engineer (let alone an electrical engineer) as one could get (for instance, my 12-year old niece). How would you explain the operation of this circuit, so that she understands why she can turn on the lights at the bottom of the steps and turn on the light at the top of the steps, using different switches but changing the state of the same light?*

For any light to work, power must flow from a power line, then through the light, and back on a return line. On a three-way setup, the light is between the two switches. The light touches the socket in two places. Each side of the socket for it has a wire to each of the switches. If both switches are in opposite settings (on and off) then one side or the other of the socket is not connected to a power or a return line. No power flows, and there is no light. If both of the switches are the same (on and on, or off and off) then both sides of the socket are connected to a power and a return line. The power flows, and the light goes on.

You need to explain the operation of a single pole double throw switch. A landing/hall light uses two of them and logically makes an Exclusive NOR gate so if both switches are up or both switches are down the light comes on. If one switch is up and the other is down, doesn't matter which one, then the light is off. So you have a simple Truth table :

Landing Sw    Hall Sw Light
Up  Up ON 
Down Down ON
Down Up OFF
Up Down OFF
  
See "2-way Lighting Circuit Diagram" at http://sparky.uku.co.uk/installations/li…

http://en.wikipedia.org/wiki/3-way_lamp