“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.”