“I’ll bet if I had some of that tracer I wouldn’t lose so many things,” Eddie said. “I could paint a little on my marbles or sling-shot. Then I could always find them with a Geiger counter.” “You could, at that,” his father said. “And I wouldn’t be stepping on the marbles in my bare feet. But, of course, great care must be taken in handling radioisotopes, which is what tracers are.” Jim had warmed up to the subject, and wasn’t quite ready to drop it. “Tracers are used in many ways,” he went on. “They are used in medicine to locate diseased tissue which attracts and absorbs certain isotopes. A radiation-sensitive instrument, similar to a super Geiger counter, sniffs out the isotope and locates the damaged tissue. Then the doctor knows what to treat, or where to operate. Radioisotopes are used in various food tests. By watching the tracer with electronic gadgets, they can tell whether the food is a muscle builder, a bone builder, or what.” “You can make machine parts radioactive,” Mr. Taylor said. “Then by seeing how many radioactive particles are in the oil after the machine has been run, you can tell how much wear the machine has taken. Oh, there are hundreds of ways to use radioactive tracers. You might call them atomic signposts. Using a Geiger counter to read the signs, you are directed along the paths that lead to the answers of nature’s mysteries.” “Wow!” Eddie exclaimed. “Pretty flowery, at that, I guess,” his father said, smiling. “Well, anyway, Jim, you did a nice job of explaining it. Now, I think we’d better get back to our work. Thanks for bringing the notebook over, Eddie—and Teena.” The two young people turned and started back toward Eddie’s house. “Let’s go across the mall,” Teena suggested. “I haven’t been over here for a long time.” The mall, as it was always called, was a broad ribbon of lawn which stretched for more than a block down the center of the college campus. It was bordered on both sides by the many buildings which made up Oceanview College. Sidewalks laced back and forth across the mall. During class changes, the area swarmed with students. Now, as Eddie and Teena walked along the mall, only a few students sauntered around or sat loafing in front of the buildings waiting for their next class. Teena and Eddie walked past the library, the assembly hall, and the nuclear-science building. They were starting past the chemistry building, when Eddie tugged at Teena’s sleeve. “Look,” he said, pointing to the back of a man walking about fifty yards ahead of them. “Who’s that?” “Who’s who?” Teena asked. “You mean that man? Am I supposed to know him?” “I think I do,” Eddie said. “He sure looks familiar.” “How can you tell? All we can see is his back. Lots of backs look alike. He’s tall. Maybe he’s a basketball player. He looks older than most students, though. Why, his hair’s even a little gray, and—”