Catalysis
Catalysis is when a substance speeds up a chemical reaction without being used up itself, like yeast making hydrogen peroxide foam instantly.
Think of it this way
A potato dropped into hydrogen peroxide makes it fizz wildly, but the potato itself stays the same size and shape. The potato has a substance on its surface that speeds up the breakdown of the liquid. Without the potato, the liquid just sits there. With it, the reaction happens fast — and the helper is not used up.
Explaining catalysis by grade level
When you pour hydrogen peroxide into a bottle and add yeast, foam shoots out fast. The yeast makes the change happen quickly instead of slowly. But the yeast itself does not get used up. It just helps. Without the yeast, the liquid would sit there for a long time before anything happened.