>Suppose that you can’t remember whether Pluto is a dwarf planet, and you need to find out by asking someone nearby—but you don’t know whether that person is a knight or a knave. What’s the one yes-no question you can ask to figure out whether Pluto is a dwarf planet?

>"Are you a knight if and only if Pluto is a dwarf planet?"

>If the person’s a knight and Pluto is a dwarf planet, then you get the answer “yes,” since both statements on each side of if and only if are true, and knights always speak truly.

I don't get how this follows. Surely, the person being a knight or a knave is independent from whether or not Pluto is a dwarf planet so a real knight would say "no" either way since the assumption is bad and a knave would thus say "yes". Thereby, you have not determined whether or not Pluto is a dwarf planet but instead whether the person is a knight or a knave.

Please explain my error.