It was an excruciating moment. A prominent BBC journalist – famous for skewering his interviewees – being totally skewered himself by his most high-profile interviewee of the week.
The guest: Britain's prime minister. The subject: the UK's referendum on changing the voting system.
David Cameron leads the "no change" camp. His central argument is that the current "first-past-the-post" (FPTP) system is simple, the proposed "alternative vote" (AV) system is not.
Famous for his argumentative style, the BBC's John Humphrys walked straight into Cameron's trap. By constantly interrupting the prime minster's quite straightforward explanation of how AV works, Humphrys made it appear complicated.
The interruptions weren't as "wrong" as Cameron claimed them to be, but the prime minister's explanation was broadly accurate, and didn't deserve loud contradiction.
But interrupting anyway, Humphrys made the AV system appear complex.
For Cameron, job done.
"That is quite worrying," he was able to crow, "the lead broadcaster of the BBC doesn't understand the system you're meant to be explaining to the public! ... [Go] back to school!"
The exchange neatly highlights the difficulty the "Yes to AV" camp has.
Their system, by any objective measure, is fairer than FPTP and it is, actually, fairly simple. But not so simple that it takes no explanation.
Their opponents have the advantage that anyone can point straight to the winner of a race.
Australia, where I'm based, has used AV – or preference voting as it's known here - since 1918.
The system for federal elections isn't identical to that proposed for the UK: voting is compulsory in Australia – it won't be in the UK – and voters 'Down Under' must rank every candidate preferencing isn't optional, as it would be in the UK.
Does it work here? Well, Australia has had stable government for decades, majority governments are the norm, and no one complains about the cost of elections.
Most of the arguments made on behalf of the UK's "vote no" camp, then, hold little sway here.
As for those who say, 'so why bother changing?' AV voting has mattered in Australia.
Last year's federal election would have seen victory for opposition leader Tony Abbott under FPTP.
Julia Gillard was able to scrape together a government because her party's candidates were less disliked by small-party voters than Abbott's.
That's a victory for candidates who can appeal beyond their base.
But do most Australians fully understand their system? Anecdotally, the answer has to be "no".
In Canberra I visited an education centre for children on "how to vote".
It takes two hours to complete the demonstration and tour the woman who runs it says they haven't enough guides to meet the demand from school groups.
If they opened it to adults too, they'd be totally overwhelmed.
There are plenty of AV explanations online, but here's mine to add to the mix:
Imagine five husbands asked to vote for the prettiest of their wives, in a public vote, while their wives were present.
First-past-the-post would result in either divorce, or stalemate. AV would find the "right answer" with ease.
I admit, I've simplified a little. But only a little.