Switches in series and in parallel

There are two ways of connecting two switches into a circuit:
(a) series – one after the other
(b) parallel – side by side

You can see the two versions in the following diagrams

(a) series circuit

The lamp will only light up when switch A AND switch B are closed (diagram 3).


(b) parallel circuit


In this circuit the lamp will light up when switch A OR switch B OR both are closed.

Try and explain why this happens.