Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Catalysts are not reactants, but they help reactants come together so they can react. You can see one way this happens in the animation at the URL below. By helping reactants come together, a catalyst decreases the activation energy needed to start a chemical reaction. This speeds up the reaction. Answer: activation energy
A catalyst decreases the amount of what resource that is required in order to begin a chemical reaction?