Conversation so far:
What is Epistemology?
Epistemology is the study of the nature of knowledge, justification, and the rationality of belief. There is debate about whether knowledge is possible at all, called skepticism.
What was Hilary Putnam's brain in a vat thought experiment?
Hilary Putnam is known for his thought experiment, arguing that one cannot coherently state that one is a disembodied brain in a vat placed there by some mad scientist.
What did Hilary Putnam's brain in a vat thought experiment prove?
If a brain in a vat were to say I'm a brain in a vat, she would be saying I'm a brain-image in a vat-image, which is incoherent.

Question: How did it explain metaphysical realism?
Self-contained rewrite: How did Hilary Putnam's brain in a vat thought experiment explain metaphysical realism?