I love "That's an interesting assertion. Why do you think masks save lives? " - it's so often a good idea to turn the question round onto the questioner, but it hardly ever occurs to me. (I think deep down I assume I'm still 13 and have to answer the questions, not ask them back!)
You're right, it's likely to lead to the topic of how much research they've done for themselves and the answer to that is - usually - almost none. It boils down to, "Well that's what the BBC/Sky/Boris says, so it must be true."
*We're* the ones who've spent so much of our scarce free time researching like manic PhD students these past 18 months... I mean, I've heard a few conspiracy theories in my time (moon landings, flat earth, Elvis Ate My Hamster) but if the "tin hatters" who spoke to me had done a tenth of the in-depth research we have, I'd have at least looked into the matter for myself. Padlocked the hamster cage, maybe.