Bill Culver Jr., age 12, of Portland, Ore., for his question:

HOW IS SOLAR ENERGY PRODUCED?

Solar energy is the energy given off by the sun. This energy is produced by atomic reactions that take place inside the sun.

Atomic reactions in the sun change hydrogen atoms into helium atoms. Every second, the sun changes about 657 million tons of hydrogen into 653 million tons of helium. The missing four million tons of matter is changed into energy.

The sun started to give off energy about 5 billion years ago. Scientists tell us that it will continue to give off energy for at least another 5 billion years.

Here is a mind boggling example of how much energy the sun produces:

Suppose that gasoline flowed over Niagara Falls at the same rate that water now flows over the falls. This is about 5 billion gallons per hour. Now suppose that all the gasoline that flowed over the falls for more than 200 million years could be collected. If all that gasoline were burned at one time, it would equal the amount of energy that the sun gives off in just one hour.

Scientists use a unit called a kilowatt to measure power. The sun continuously produces 390 sextillion kilowatts of power. This number is written by adding 21 zeros after the number 390.

The production of solar energy is a continuous thing by the sun. But the energy is sent out in every direction. As a result of this, only about 1 two billionth of the sun's energy ever reaches the upper atmosphere of the Earth.

Even so, the earth each year receives more than 1,500 quadrillion kilowatt hours of power from the sun. This number is written by adding 15 zeros after the number 1,500.

Not all of the solar energy that reaches the upper atmosphere of the Earth reaches its surface. Only about 47 percent of the solar energy makes it through the atmosphere.

Here's another mind boggling example of solar energy:

Imagine a loaded coal train that is long enough to reach from the Earth to the moon and back five times. If all of the coal of this long train were burned at one time, it would produce the same amount of energy that reaches the upper atmosphere of the Earth from the sun in just one hour.

More than 700 quadrillion kilowatt hours of power reach the Earth's surface every year. Man uses more than 50 trillion kilowatt hours of power to run his factories, machines and vehicles and to heat his buildings.

This figure shows that in just 40 minutes of time, the sun delivers to the Earth's surface as much energy as man uses in an entire year.

All plants and animals on the Earth need the energy of the sun to live. Green plants, by means of a process called photosynthesis, store solar energy as food. With photosynthesis, solar energy is changed to chemical energy by plant cells.