Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: What is one of the most important characteristics of a referee? A referee must be neutral. He can't favor one team over the other. In chemistry, being neutral means not being an acid or a base. Pure water is an example of a neutral substance. In some chemical reactions, an acid and a base combine to form neutral products, including water. You'll see how this happens when you read this article. Answer: neutral
A:
What do you call a substance that is not an acid or a base?