For a long time, our society has been relying on sources of energy with limited amounts. These are called nonrenewable energy sources because they cannot be replenished within a short period of time. If we continue to use nonrenewable energy sources, our supply will run out someday.

PETROLEUM: Oil wells pump out the petroleum from tiny pockets underground. When the wells run dry, there will be no more petroleum.

COAL: Mines are set up underground so people and machines can dig out the coal. Once all the coal is mined, there will be no coal left.

NATURAL GAS: Drilling is necessary to reach the natural gas trapped in pockets underground. Once all the gas is released, there will be no more.

PROPANE: Power plants and oil refineries separate out propane from petroleum and natural gas. When these things run out, so will propane.

URANIUM: Open-pit mines are dug into the Earth's crust to extract rocks that contain uranium. Once all the uranium is mined, it will be a depleted energy source.