Why does a person's mass stay the same when he travels to the moon, but his

weight changes? Give a detailed answer that includes scientific terms.

No matter if you are on Earth, the moon or just chilling in space, your mass does not change. But your weight depends on the gravity force; you would weigh less on the moon than on Earth, and in space you would weigh almost nothing at all. Where ever you are, your mass will always stay the same.