Kathy Coyle, age 10, of Staten Island, New York, for her question:     

What is convection?

Convection is a movement, the special movement of a current of heat. You need some deep thoughts to understand what causes even a simple movement of your hand. Heat is even harder to understand. So before you tackle the answer to this question, be sure to put on your thinking cap.

Before we can understand convection, we must know a bit about heat and movement. We can feel the results of heat and see the results of movement. But these mysterious energies act behind the scenes, and it is easier for us to understand things when we can watch how they work. However, our clever scientists have solved the major secrets of heat and" movement which they call motion. Their answers are not as simple as ABC, but we figure as that when you are old enough to ask a question, you are ready for the answer. And your faithful old reporter has trained himself to explain tricky problems in words that curious young people can understand and, he hopes, enjoy.

Let's begin with heat. The experts say that heat is a form of energy. This means that it has the pap to perform something, such as warming your hands or boiling water for a cup of tea. Heat can do things with its energy. But like everything in nature, it must obey a set of strict rules. For example, it can and it must_give its heat energy away. It must transfer, or move, its heat to objects around it. A hot stove must give heat to a pot sitting on the flame. A hot radiator must transfer heat to warm the air around it in a room.

The transfer of heat from a warm object to cooler objects around it calls for movement or motion. This motion of heat also must obey rules. And one of these rules is a traffic system called convection. Most substances swell up and get a bit bigger as they get warmer. And all substances, whether they are solids, liquids or gases, are made of tiny atoms and molecules. When a substance gets more heat energy, its tiny particles move faster and spread out, leaving more space between them. This is what makes them expand.

When a substance warms up, it becomes lighter because there is more space bettaeen its particles of matter. When it cools, its atI-

_“noms and molecules lose some of their heat energy and crowd more closely together. It becomes denser and heavier. You know that a cork bobs on the waves because cork is lighter than water. Light, warm things and heavier cool things must obey rules somewhat like the rules of floating and sinking. Warmed air in a room tends to rise as it expands with heat energy. Cooler, heavier air in the room tends to sink down through the light, warmer air.

These movements of heat are called convection currents. A stove heats the lowest level of water in a saucepan first, because the bottom of the pan is closest to the flame. The warm bottom layer spreads out and gets lighter. This makes it rise up through the cooler, denser water on top. As the hot water bubbles up, the cooler water sinks down. Convection currents mix up the layers of heat until all the water in the pot reaches the sane temperature.

Convection currents of heat work on a grand scale in nature.. Musses of warm air spread out and rise through masses of cooler air. Convection currents in the atmosphere create winds and clouds, calm spells and weathery storms. Other convection currents move around in the deep insides of the solid earth. Modern scientists suspect that they carry heat from the center of the globe up to the surface. Some experts think that these deep, slow convection currents may crack the earth's crust and cause earthquakes and volcanoes.