When you write a research paper, why should you never choose a topic on which you're already
an expert?

You shouldn't choose a topic that you're already an expert on for several reasons. First off, you aren't learning anything. The whole point of a research paper is to learn something new.

Secondly, you could gloss over facts. If you already know a lot about a topic, it's easy to assume that the person reading your work knows the same amount as you, but that isn't always true, so it might not make sense to them and they might not understand

the answer is trees if i'm not mistaken