"I know what it is, but I don't know why it's so important to Steve," Rick said. He had read a great deal about heavy water in studying elementary physics. It had many uses in physics experiments. "Let's see how much you know," Steve directed. "Sound off." Rick searched his memory, trying to marshal all the facts he knew. "Well," he began, "ordinary water is composed of oxygen and hydrogen. In every water molecule there are two atoms of hydrogen and one of oxygen. The important part, for what we're talking about, are the hydrogen atoms. Hydrogen is the lightest element, and it has the simplest atom. There's just one proton and one electron." He looked at his father, waiting for a nod to tell him he was on the right track. When the scientist nodded approval, he went on. "That kind of hydrogen atom has a mass of one, as the scientists say. But there are other kinds of hydrogen atoms, and they are pretty rare, called isotopes. An isotope is just a different variety of the ordinary kind of atom in each element. The thing that makes it different is a change in the nucleus. Well, hydrogen has two isotopes. One kind, which has a mass of two, is found in nature. It is called deuterium. Its nucleus is called a deuteron. Another kind, which can be made in a nuclear reactor, is called tritium. A little of it is found naturally but not enough to count for much." He took a deep breath. "I hope I know what I'm talking about." "You're doing fine," Hartson Brant said. "Go on." "All right. Well, heavy water is made of one atom of oxygen plus two atoms of deuterium, which is the first isotope of hydrogen. In chemistry, there's no difference in the way heavy water acts. You can even drink it. In fact, people do drink it every day, because in ordinary water there is some heavy water. I forget the exact figures, but I think that, by weight, there are five thousand parts of ordinary hydrogen in water and only one part of deuterium." "That's right." Steve Ames nodded. "Five thousand to one. Now tell us what is peculiar about all isotopes?" Rick thought furiously and came up with what he hoped was the answer. "I think it's that isotopes aren't as stable as the basic elements. Some are pretty stable, but some are pretty shaky. That's why some of the isotopes of uranium can be split wide open in a chain reaction to make an atomic bomb, and ..." A chill ran through him. His mouth opened. He knew! He knew why heavy water had Steve Ames all excited. He choked: "Hydrogen bombs!" Scotty and Barby gasped. Steve Ames and Hartson Brant smiled. "It's true that one of the possibilities in building a hydrogen bomb concerns deuterium," the scientist said. "But I scarcely think that's the case here. How about it, Steve?" "Possible, but extremely improbable," Steve agreed. "What I'm most interested in is a use for heavy water Rick hasn't mentioned. Know what a nuclear reactor is, Rick?"