When a little kid asks “why?” five times in a row, they’re not being annoying. They’re doing research.

Repeated questions are a child building a model of how things connect, and the fifth why is usually the one that reaches the part you cannot explain either. That is not defiance, it is the only method available to somebody with no reference books and a lot of gaps. The irritation is often about being found out rather than about being asked. Children stop asking when asking reliably annoys people.

Answer as far as you honestly can, then say you do not know, which is true and worth modelling. Look it up together if you have ten minutes. Ask them what they think the reason is.