David Cameron tiptoes around something very serious.
It's not something politicians do much. They barrage each other with insults, of course, and they describe each other - in public at least - in highly unpleasant terms, but they rarely get into really serious business.
Mr Cameron went there today. He asked Gordon Brown, once more, if he would confirm he had not made any offers or deals to win the 42-day detention vote.
"Yes," Mr Brown said simply.
And then Mr Cameron did something interesting. He brought up a letter, published in the Daily Telegraph, from Geoff Hoon, chief whip, to Keith Vaz, chair of the home affairs select committee, which ended: "I trust it [his help] will be appropriately rewarded!"
Mr Vaz led objections to the plans but changed his mind in the face of various government concessions.
Suddenly, Mr Cameron was coming very, very close to calling the prime minister a liar. That's just not done. That's serious business.
For example, even MPs politics.co.uk has talked to in the past who called for Tony Blair's impeachment over the Iraq war never called him a liar, and certainly not anywhere near media or parliament.
Mr Cameron obviously didn't put it that way. An "utter inability to be straight with people," he called it. But everyone knew what he was saying. Journalists had been thinking it since a day of watching DUP MPs scuttle from the whip's office and back was followed by a press conference in which Mr Brown promised no deals had been struck. It just seemed implausible. The press know that. The Tories know that. And you can bet your last bank note they are doing everything they can to find some evidence somewhere.
Mr Brown had just enough time to call Mr Cameron's bluff. "If he has any allegation to make." he started, before the speaker moved the debate on.
That in itself was an interesting development. The speaker, Michael Martin, can call order on anyone, of course, but it's rare for him to stop the prime minister when he's answering a question at what is, after all, called prime minister's questions. One can't be categorical, but the impression that Mr Martin saw things getting serious and pulled the plug lingered in the air for the remainder of the session.
It was the most interesting moment in an otherwise fairly pedestrian PMQs. Mr Cameron made some headway in his second line of questioning, on the early release of prisoners. Nick Clegg continues to be competent and entirely unremarkable. A senior political journalist politics.co.uk talked to last week admitted sometimes not being able to remember his name.
Mr Clegg is very justified in attacking Mr Brown's tendency to confuse lists with answers, although by preceding that point with "he's doing it again" he provoked only cat-calls and Benny Hill-era comedy from the rest of the House.
Mr Brown is still learning how to do these things, but time's pretty much run out now. The stammer is still there when under pressure, and the overuse of phrases like "I have to say to him" is starting to grate.
Given the result in Henley, though, Mr Brown was lucky to get away from today's meeting as easily as he did. The speaker probably had more than a little to do with that.