Radioactivity is very interesting, but before we can understand its real importance, we must learn a little about energy. To most of us, energy means "pep." To a scientist, energy means _the ability to do work_. Energy is not a "thing." You can't see it. You can only see--or hear--or feel--what it does. Energy never disappears, but it can be changed from one form to another. When you swing a bat and wallop a ball, part of the energy you use makes the ball whiz through the air. If you use energy to clap your hands, part of the energy is changed to sound, and you hear a noise. If electrical energy is used in a light bulb, part of the energy is changed into light and part into heat. [Illustration] When we burn wood for heat, we are using energy that the tree took from the sun. When we burn coal or oil, we are using the energy of sunlight that was stored many millions of years ago. All of this energy is stored in the atoms of the wood, coal, or oil. But when we burn these materials for fuel, we release only the energy of the _electrons_. Now do you remember, back in the last chapter, we said that the nucleus is the heavy part of the atom? And that the electrons are very light? Well, the nucleus is so very, very heavy for its tiny size, that it cannot be compared to anything else in the world. If a nucleus were as large as a grain of rice, it would weigh two million tons! Nothing so small could weigh so much unless it were extremely tightly packed together. It takes a great deal of energy to pack anything that solidly. By the middle of the 1930's, scientists were beginning to think about the huge amount of energy that would be released if the nucleus could be split. The scientific name for splitting is _fission_. Just suppose, the scientists thought, you could split a nucleus and its neutrons would come flying out--and each neutron would strike like a bullet at another nucleus and make that one split? And all the new flying neutrons would split other atoms? This would be a _chain reaction_. If man could produce a chain reaction, there would be such energy as the world never dreamed of! In many different countries, men thought, and dreamed, and worked--the search for the nuclear chain reaction was on! 6 JOURNEY TO THE NEW WORLD It was a gray winter morning. The date was December 2, 1942. The place, The University of Chicago. Here at Stagg Field, under the football stands, was a large empty room that had once been a squash court. None of the students who hurried by on the way to class paid much attention to a few men who passed through the door into the long unused room. No one knew that in that room one of the greatest events in the history of science was about to take place. No one knew that the atomic age would be born that day.