Gurleen Sikand, age 15, of Winnipeg, Manitoba, Canada, for his question:

What does the formula for energy mean?

Suppose you wished to write a dictionary definition for the word energy. The project sounds simple. After all, we talk about energy almost every day. Surely we should know what it means. However, when we get right down to defining it, we are not so certain. There are many kinds of energy    such as fuel energy, muscle energy and nuclear energy. What's more, one form changes to another. For example, the fuel energy in coal can turn a generator to create electrical energy.

One of the best brain stretching exercises is to think about the various aspects of energy. It is, of course, silent and invisible. Basically it can be defined as the ability to do work. We can measure the amount of work energy in units called foot pounds. One foot pound is the energy used to lift one pound a distance of one foot above the ground.

Let's explore this further with a pound of sugar. You use a foot pound's worth of muscle energy to hoist it onto a ledge that is one foot above the floor. In some strange way, your muscular effort now belongs to the bag of sugar. It can be used if the sugar falls off the shelf and lands on the floor. This expenditure of activity is called kinetic energy, which is energy in motion.

Now let's go back to where you got the original muscle power. Naturally it came from your food, directly or indirectly from the plant world which depends on energy radiated from the sun.

The sun is, as we know, an enormous nuclear powerhouse that fuses atoms of hydrogen to create atoms of helium. In this process, some of the sun's material is converted into energy    the solar radiation that sustains all life on our planet. In all forms of nuclear energy, atomic particles are converted into energy.

Now we know that mass and energy are two sides of the same coin. Under certain conditions, mass can be converted into energy or energy into mass. This is stated in the brain straining formula E me squared. The E stands for energy and the m means mass, which is the amount of matter in a given volume. C squared means the velocity of light multi¬plied by itself. The formula boldly states that mass and energy are related and defines this relationship in terms of how much mass equals how much energy.

In the past, it was assumed that the total energy in .the universe cannot be changed. This was stated as the conservation of energy. We now know that, though the energy in the universe cannot be destroyed, it can be converted from matter    and matter can be converted into,,. energy. In terms of the universe, this is stated as the conservation of mass energy.