Here's why:

* Causes or speeds up a reaction: Catalysts lower the activation energy required for a reaction to occur, making it happen faster or even allowing it to happen at all.

* Remains unchanged: Catalysts participate in the reaction but are not consumed in the process. They can be used again and again.

Examples of catalysts:

* Enzymes: Biological catalysts that speed up reactions in living organisms.

* Metals: Platinum and palladium are used in catalytic converters in cars to speed up the conversion of harmful gases into less harmful ones.

* Acids: Some acids act as catalysts in chemical reactions.

Let me know if you'd like more information about catalysts!