If a circuit has 2 batteries, you can connect them using a series connection. In a series connection, the negative (-) end of one battery must be connected to the positive (+) end of the other battery. For the light bulbs to go on, electricity must flow from one end of the connected batteries, through both bulbs and the switch, to the other end. The switch turns both bulbs on or off. If one bulb burns out, the other will also go off because the circuit will be broken.
