To break down means to stop working or functioning properly. When a car breaks down on the highway, its engine quits and it can't move anymore. When a washing machine breaks down, it stops cleaning clothes. Usually when something breaks down, it needs to be repaired before it can work again.

The phrase also means to separate something complicated into simpler parts that are easier to understand. A teacher might break down a difficult math problem by showing each step separately. When you break down your weekend chores into smaller tasks, you make a big job feel more manageable. Scientists break down complex questions into smaller experiments they can actually test.

People can break down emotionally too. When someone breaks down crying, their feelings become so overwhelming that they can't hold them back anymore. This kind of breaking down isn't about weakness: sometimes emotions build up until they need to come out.

In a different sense, materials break down when they decompose or fall apart over time. Leaves break down into soil. Old paper can break down and crumble. This natural process returns materials to simpler forms, like how a cardboard box eventually breaks down in a compost pile.