With straggly grey hair and a well lived-in anorak, he sure looked the part. It is more than three decades since I encountered my first properly mad crank.

This was on an inter-city bus somewhere in southern Russia. There was no way to escape him: he had a theory, a wonderfully crazy one, and he had a captive audience for it.

And so for an hour or so I and a clutch of passengers had to listen as this eccentric gent explained, with an educated tone that belied his appearance, that governments were lying about water.

The stuff that comes out of the taps, he said, was not, as the authorities claimed, made of three atoms, two hydrogen and one oxygen.

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No, the man explained, water was a polymer whose remarkable properties were being hidden for, well, reasons I never quite understood.

Now this was around the time the Soviet Union was collapsing – and with it a lot of old certainties, old faiths, old trusts.

Amid all the turmoil some folk just “lost it” – rather like we saw here during the pandemic, Brexit and indyref but on a – I think – bigger scale.

In early 1991, by way of illustration, there was a TV show that claimed Vladimir Lenin, the pointy beard behind the October Revolution, had done so many magic mushrooms he had turned in to a fungus.

So the “water is a polymer” guy was kinda on-brand for his time, not least thanks to an enduring fad for “natural” spring water.

But over the last decade and a half or so of social media-driven politics, I keep finding myself thinking back to that first proper zoomer encounter.

Because there was a lad on the bus who knew his chemistry: and quietly and politely he explained that water was H2O and there was no way this simple compound was a big chunky polymer.

Or to use the terms of our social media age, the anorak on the bus was fact-checked, community-noted, and found wanting.

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Don’t get me wrong: I am sure the gent went home every bit as confident in his theory of jumbo molecules in his shower as he was before his ride on the coach.

But his ability to persuade others was limited, severely so.

The “chemist on the bus” – in my head, anyway – has been how we have chosen to tackle a whole range of nonsense online.

Since the forerunner of Snopes appeared way back in 1994 to counter internet folklore we have tried to “fact-check” our way out of a crisis of misinformation and disinformation.

But what works on a bus – or in the pub, the classroom or work meeting – does not necessarily have the same impact on social media.

I think we all know the limits of internet fact-checking, even when we are talking about really simple incontrovertible facts, like the chemical composition of the most common liquid on the planet.

The good news is that there is not, as far as I can tell, a large internet gang of water-deniers. But we do now have online wackadoodles who support each other in beliefs that are impossible to reconcile with objective reality. So there are people who think the earth is flat, birds are drones and, yes, that Vladimir Putin, like Lenin, is a mushroom.

And, worse, we have communities of partisans and hyper-partisans who cynically indulge, encourage and disseminate myths or even straight-up lies as a kind of badge of membership or weapon of hate.

Take one of the most salient misinformation narratives in Scotland: that Scots is not a language, despite the fact that it is recognised as such by just about every possible authority, government or academic.

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Now there are going to be a lot of people who don’t particularly care about the status of Scots. And that is a pretty healthy approach to take.

But there are obviously very online activists who find the reality of Scots as a recognised language objectionable, perhaps because they fear this challenges their ideology of a single indivisible British nation. So they just reject the linguistic science, or even the evidence of their own ears.

It helps the deniers that we live in a society with low levels of general language knowledge and a long tradition of minoritising and demeaning indigenous speech. There are stupid, long-learned prejudices – and brute ignorance – out there to be exploited on this topic. Last week somebody posted that only "at core, fascists" thought Scots was a language. That is how low this online discourse has fallen.

But surely linguists – like my chemist on the bus – could “correct” online activists? Cannot experts ask Scots deniers to explain why they are right and, say, Unesco, is wrong about what constitutes a language? Well, they try. But it does not necessarily work.

Because the haters do not care if what they say is true or not. They know abusing Scots speakers gets a rise and they wear their lie like the Union flag in their avatar: it tells people what tribe they belong to, it tells their peers they are willing to sacrifice their credibility for their flag. Moreover, there are internet entrepreneurs – hungry for cash or clicks – who will help them do so.

Fact-checking: it does not always work.

That is why in recent years we have seen a new way of looking at misinformation as a symptom not a disease.

So, for example, in this diagnosis Scots language denialism stems not just from ignorance about linguistics but also from tribalism, political polarisation, and a lack of faith in institutions.

And that is where I find myself thinking about the “water is a polymer” guy and the lad who corrected him. Misinformation – about chemical or linguistic science or anything else – thrives when trust dies. As in Russia 30 years ago. Or, to some extent, in some western democracies now.

We do need fact-checking, of course we do.

But we also must ask our opinion leaders – politicians, journalists and even social media influencers – to tackle the underlying distrust and division which enable ubiqituous BS. Are they willing to do so?