Nuclear Chain Reaction
A nuclear chain reaction is when splitting one atom releases energy that splits more atoms.
Think of it this way
A row of mousetraps sits on a table, each one loaded and ready. You drop one ball bearing onto a trap, and it snaps — flinging two balls into the air. Those two balls each hit another trap, which flings two more balls each. In seconds, every trap on the table has fired, filling the air with flying balls.
Explaining nuclear chain reaction by grade level
Think of a long line of dominoes. You push the first one over. It hits the next one, and that one hits the next. They all fall, one after another. That is how a chain reaction works.