Assume that you are a deeply ambitious politician. You want to be prime minister, but you are not leader of your own party yet. Nonetheless, rumours are circulating in Westminster that you are lining up for the top job. How to respond?
That is the current predicament of London's mayor, Boris Johnson, which he handled with customary panache when he gave ITV News his thoughts on leading the country one day.
It is one of those "damned if you, damned if you don't" questions. If you say yes, you want to be prime minister, you risk sounding disloyal to your leader and, worse than that, you could be inviting ridicule. Robert Kilroy-Silk made that mistake when, days after being elected, he announced that he hoped to be prime minister within 15 years. Sixteen years passed and Kilroy-Silk, having reached the middle ranks of the Opposition front bench, got fed up with the taunts and quit the Commons.
But if you say "no", potential supporters might think you mean it and drift away. If you refuse to answer at all, you can sound like you cannot make up your mind. That was the problem that bedevilled David Miliband as he mulled over whether he should try to depose Gordon Brown from the premiership.
The trick is to change the subject. In June 1974, when rumours were gathering pace that Margaret Thatcher might challenge Edward Heath for the Tory leadership, she gave the Liverpool Daily Post this classically evasive answer: "It will be years before a woman either leads the party or becomes Prime Minister. I certainly do not expect to see it happening in my time."
Michael Heseltine was equally adroit in March 1990, when Thatcher's dwindling supporters demanded a declaration of loyalty from him: "I have repeated many times that I cannot see circumstances in which I would challenge Mrs Thatcher." Johnson has used the same technique, usually making a joke by suggesting that he is more likely to be reincarnated as an olive. This week, he paid homage to Thatcher's old foe by saying: "In the immortal phrase of Michael Heseltine, 'I cannot foresee the circumstances'."
What he did not say, but others will have remembered, is that Heseltine used the phrase for the last time in November 1990. But instead of saying "cannot", he said "could not" as he launched the leadership contest that ended Thatcher's career.