Humans have invented thousands of machines and appliances that utilize energy to make the daily works easier, for instance to heat our house, to get ourselves from place to place. Some of this machines use electricity, while others, like automobiles use the energy stored in gasoline. Much of the energy supply comes from coal, oil, natural gas, or radioactive element. In fact, all these natural resource deposits took millions of years to form. They are considered non-renewable which means once they are removed from the ground, they are not immediately replaced within the human timescale.