How does a slow-rolling steel ball launch another ball fast enough to blur in a photo? A magnetic chain reaction does the work. Each magnet in the line adds energy to the next ball. You tape four strong magnets along a grooved ruler. Two steel balls sit to the right of each magnet. Roll one ball slowly toward the first magnet. It snaps in and transfers energy through the group. The last ball shoots toward the next magnet even faster. The chain reaction builds speed at every stage. The final ball fires off the end of the ruler and hits a target hard.