This circuit includes two small incandescent bulbs, a 3.5 volt "D" battery, and a switch. It is wired in series, meaning the electricity flows through the first bulb, and then the second bulb, before it gets back to the negative side of the battery. This type of circuit requires more voltage, causing the bulbs to either glow dimly, or not turn on at all.

This is the same circuit wired in parallel, which means that the electricity flows through both bulbs at the exact same time. This allows both bulbs to get full voltage from the battery, but will drain the battery faster, as it is pulling an increased amperage as well.