Electricity passes easily through metals like iron and copper. But electricity does not pass through wood, rubber, glass, and other materials. What is the difference that causes electricity to flow in some things, but not in others? The reason why electricity flows in copper wire and other metals is because metals have electrons that can move around freely inside them. These electrons are called free electrons. Batteries have an electric force inside them. So when a battery is connected to a metal wire, its electric force causes the free electrons inside the metal to move together in one direction. The free electrons move from the negative end of the battery to the positive end. This movement is the flow of electricity. On the other hand, wood, rubber, and other materials do not have a single free electron. That’s why electricity does not flow in these materials.