“I’m afraid that’s not too clear, Jim,” Mr. Taylor said. “Teena’s not an atomic scientist. Can you simplify it for her?” “Oh, I know what a radioactive isotope is, Mr. Taylor,” Teena said proudly. “You do?” Eddie’s father said in surprise. “It’s something—I think you call it an element—which they put in an atomic reactor, and—and cook it until it becomes radioactive. Kind of like a sponge soaking up water.” “Very good,” Mr. Taylor nodded, obviously impressed. “Eddie explained it to me,” Teena said, smiling, “but he didn’t say anything about tracers.” She turned her attention back to Jim, the student. “All right,” Jim said, seeming more at ease, “let’s look at it this way. Any radioisotope keeps shooting out rays. Of course, you can’t see the rays with your eyes. They’re almost too small to think about. But you can follow them with a Geiger counter.” He indicated the black metal instrument which he still held in his hand. “Well,” Jim went on, “say, for instance, that you wanted to know how fast a stream of water flows. You might toss in a rubber ball and time how long it takes to float a mile downstream. That would give you its speed. Or say you wanted to know which way its currents twist and turn. You might dump in a gallon of ink and watch it follow the currents. In a way, the ball and the ink are tracers. Not radioactive tracers, of course, but by watching how they act, you learn what you want to know.” “Let’s talk about radioactive tracers,” Eddie said eagerly. “You’re crowding me, bub,” Jim said, smiling. Everyone laughed. “How are we using tracers here, Jim?” Mr. Taylor prompted. “We’re testing the use of phosphate in plant growth,” the student explained. “We want to know what the plant does with it. Does a phosphate fertilizer merely feed the plant’s roots, or is it pulled up into the stems and leaves? And we want to know how quickly the plant absorbs it, if at all. Of course, we can’t see it, but if we make the phosphate slightly radioactive, then we have what we call a tracer. By using a Geiger counter, we can follow or trace its movement.” “Can you explain our method, Jim?” Mr. Taylor said. “Well, we spread a little of the radioactive phosphate around the plant,” the student said. “Soon the roots start taking it in.” “How do you know that, Jim?” Mr. Taylor asked. “We hold the Geiger counter to the root. If it starts clicking faster than usual, we know the root has absorbed some of the phosphate tracer. We also hold the Geiger counter over the stems and leaves. As the tracer works upward into the plant, the Geiger counter reacts to it. Here, let me show you how it works on this cotton plant.” Eddie and Teena moved over closer to the two-foot-high plant. Jim switched on the Geiger counter. Eddie saw the needle on the gauge flutter slightly, indicating the normal cosmic-ray background count.