It doesn't seem like there's any good way to handle armchair "philosophers" who think they have something deep to say about the idea of certainty. "See, the thing is, you can't actually know anything for sure! I mean, like, what if we're all just brains in vats?" The only compelling argument I see for this point proceeds by example: The claim is that nobody can truly know anything whatsoever. Anybody who can in all sincerity make this claim must know very little -- perhaps little enough to serve as supporting evidence.
Joking aside, this is one of the silliest ideas out there. Those who contract it generally do so within a year of their first serious exposure to the great philosophers, in that honeymoon phase where everything makes sense and the ideas of "discarding all assumptions" and "questioning everything" still seem novel. Socrates and Descartes are particularly severe offenders, as brief or unconsidered treatments of their works can easily give the impression that they advocated this total uncertainty. What, more precisely, is this misconstrued idea, though?
It's the idea that we just can't know anything for real, man. You know? Like, think about it -- everything we know comes from observing the world, via our senses, right, and how can we trust our senses? What if we all see, like, different colors? What if two plus two doesn't always equal four? What if it could equal five, but we don't believe that because we've never seen it? Or because we can't see it? Would we even know how to count if we were all blind? I, like... I dunno, dude. I dunno. Whoa. We don't even know that. We don't know anything. Not really.