The food we eat provides our bodies with the energy we need. The energy required for the train to move comes from the electricity in the overhead lines. In one case, it's entirely necessary for our survival, and in the other case, it's potentially hazardous if mishandled. Energy can be described as the ability to do work. It can never be destroyed, only transformed between different forms.

A significant portion of the energy we use comes from the sun's rays. The sun sustains plant life, drives winds, and causes evaporation from the oceans, which later forms rain. From plants, we can produce fuel to power engines, and from the movement of winds and oceans, we can generate electricity. The sun's rays could theoretically meet the earth's energy needs thousands of times over and never run out.

However, other energy sources we use will eventually run out. Coal, oil, and natural gas are fossil fuels that emit dangerous substances into the environment when burned. Nuclear power is chemically bound energy with hazardous waste as a byproduct.