We just learned a little about USB.
If you look close on a battery, you'll see a plus + for positive on one end and a minus - for negative on the other end.
(From: Wikipedia - battery)
When you connect a wire or a light bulb to the positive and negative ends of the battery,
the electricity will flow through the loop or circuit from the negative end to the positive end and light up the light bulb!
(From: Wikipedia - electrical circuit, light bulb)