A chain reaction is a sequence of events where each event causes the next one to happen, like dominoes falling in a line. When you knock over the first domino, it hits the second, which hits the third, and so on. Each falling domino triggers the next until they've all toppled.

In chemistry and physics, certain reactions create products that trigger more of the same reaction. Nuclear power plants and atomic bombs work through chain reactions: when one atom splits, it releases particles that split other atoms, which split still more atoms, releasing enormous amounts of energy in a rapid cascade.

But chain reactions happen in everyday life too. Imagine one student starts laughing during a quiet moment in class. Their laughter makes others laugh, and soon the whole class is giggling uncontrollably. Or consider how a single unkind rumor can spread through a school, with each person who hears it telling someone else, causing more harm with each retelling.

Chain reactions can be positive or negative. One person's act of kindness might inspire others to be kind, creating a chain reaction of good deeds. Scientists and engineers often study chain reactions to understand how to start helpful ones, stop harmful ones, or control powerful ones like nuclear fission.