See, first, just answer the question. It’s a simple question; you should be able to answer it pretty quickly: if I have the steel key, as well as a steel coin (and a gold coin, and handful of sand), and I note that I’m happy to give you the key, how would you reply if I ask whether I should instead hand you the coin or the sand?
Answer that, and then discuss the relevance to their point. Tell me whether you’d shrug and say “oh, well, it’s all the same thing,” and then tell me whether you still think they’re right about how it’s all the same thing. Tell me if you’d be equally glad to get handed a coin or the sand or that key (with a response like “oh, hey, handing me the coin is handing me the key — and handing me the sand is handing me the coin and the key”); and, one moment later, then do the rest.
This isn’t me trying to task you with some long assignment; it should be the work of a moment: say you’re locked in that cell, and I can give you the key that’s in my right hand; or I can give you what’s in my left hand, be it a coin or some sand. Don’t tell me what occurs to you ‘the more times you read the analogy’; feel free to spend less time just putting a reply out there first. You can do this.