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So, what did he mean by that?

David Cameron's admission that he will not serve a third term in Downing Street will provoke a flurry of speculation. What was he hoping to achieve? What message was he trying to send?

First things first. I asked him a question and he answered it. It was not something that a helpful Downing Street official had suggested I might ask with a heavy hint that I might get an interesting answer. It was just one of many speculative questions that political journalists like me ask in the hope that just occasionally they might get an answer. And this time it did.

Second, Mr Cameron's overt aim was to get across the message that he would serve a full second term. He wants to quash speculation that he might stand down early in 2017 after a referendum on the UK's EU membership.

But by emphasising that he would do another five years, he inevitably has to address what he would do after that. And his answer was clear. Terms in Downing Street, he said, are like Shredded Wheat: "two are wonderful, three might just be too many."

But by answering my question, Mr Cameron has potentially opened a Pandora's Box.

He has invited Westminster and the country to contemplate a time when he is no longer prime minister and that is a dangerous gamble to make so close to an election.

His aim is to give himself the time and space to finish a job that he told me he feels is half done. He is telling the voters that they can back him one last time in May and then they can see the back of him in five years' time. It lets voters know that unlike some of his predecessors, he will not go on and on.

But his opponents will say he is making an arrogant presumption about the election result, an assumption that he will have the choice over whether to serve a third term. In a few weeks' time, the voters may make that decision a little earlier for him.

And even if Mr Cameron does remain in Downing Street, he has officially opened the door to what would be a lengthy leadership contest. He did not need to tip some of his potential successors but he did. And I am not sure Theresa May, George Osborne and Boris Johnson will thank him for it.

So the prime minister was asked a question, he answered it, and the consequences will help shape an election campaign that is just days away.