Given a scientific passage and an answer, generate a question for the given answer.

Passage: Some reactions need extra help to occur quickly. They need another substance called a catalyst. A catalyst is a substance that increases the rate of a chemical reaction. A catalyst isn't a reactant, so it isn't changed or used up in the reaction. Therefore, it can catalyze many other reactions. Answer: a catalyst
Some reactions need extra help to occur quickly. they need another substance called what?