Action-reaction

A chain reaction is when one thing happens, and that makes something else happen, and then something else, and so on. It is like when you pull the first piece of a chain, then that piece pulls the next, and then the next, and so on.

The same idea applies to dominoes. If you push over one stone, it pushes over the next, and then the next, and so on. Sometimes things can happen very quickly, when a chain reaction starts.

In biology, a chain reaction is like when one cell in your body gets sick, then it can make other cells sick, and then more, just like an itch spreading.

And in chemistry, imagine adding a bit of a corrosive substance to a mixture. That can change the mixture in such a way that it affects other substances in the mixture, and then those in turn start affecting other substances. It's like when you add a little colour to water and it completely changes!