Aquires energy in the form of sunlight. All mechanical systems also require energy to function. A car needs gasoline to run. A sailboat needs wind to move across the water. Energy, in one form or another, is needed for all living and nonliving activity on the planet. Energy does not actually exist as a thing itself, however. Instead, energy is an idea describing various sources of power.

ll living systems require energy to survive. A person requires energy in the form of food. A plant re Long ago, humans relied upon the natural systems of the earth to meet their energy needs. Cliff dwellers of the Southwest built their homes to capture the heat of winter sunlight. Ancient Greeks bathed in water warmed by geothermal vents. Humans around the world used wood to cook their meals and warm their homes. The natural...