Heard the one about the politician on your doorstep?
"A politician who?"
"A politician who has turned up on your doorstep."
That is not really very funny, but it is not really a joke.
Yet many people do laugh when you ask them if a politician has turned up to talk to them, let alone listen - not laughing at the politician, but laughing at me for asking the question.
You don't have to go far these days in Westminster to encounter deep angst about the shakiness of the main UK parties' connection with voters around the country.
As the SNP scoops up members in Scotland at a rate of knots, UKIP looks like smashing up the Conservative kitchen sink in Rochester next week, and the Greens find a new zeal, Labour, the Conservatives and the Lib Dems wring their hands.
What was a two-party system, that became a three-party system that guaranteed them safe passage through their political careers, threatens to splinter beneath them.
And what are the parties who are making them feel so nervous doing?
Talking to voters. Holding public meetings. Opening offices in high streets. Going out and knocking on doors that haven't been knocked on in a long time.
In the run up to the Scottish referendum, I lost count of the number of times that people told us that "Yes" campaigners had been everywhere, and Better Together was nowhere to be seen.
They weren't ultimately successful of course, but there was no question about which side had the political energy, which is arguably now flooding to the SNP.
That support could fatally undermine Ed Miliband's efforts to make it to Number 10.
Politicians appear to know it is the daring act of actually talking to voters that tends to go down well.
When things go wrong, they make well-meant vows to listen to the public, to emote "I get it" after an embarrassing vote or two.
After a dreadful couple of weeks, on Thursday Ed Miliband again promised that his party would go "door by door" to sell the Labour message.
But perhaps what is really telling is that, six months out from an election, he seems to have had to remind his party, indeed himself, that it is worth doing.
Talking amongst themselves is not enough for any party.
And crucially, it is not just hearing the unvarnished truth - what voters think of them - but asking genuine questions about their constituents' lives and, guess what?, listening to the answers.
Right now, all too often that just does not happen and, more to the point, when it does, those views are sometimes dismissed.
One close, wise observer of Westminster, who knows MPs well from right across the House, tells me: "I was amazed by the readiness of some MPs to refer to their constituents as though they were an irrelevance or, still worse, as though their views were commonly ill-informed and to be disregarded."
There are of course exceptions, across all parties, and it shows:
- Jim Murphy for Labour, whose "Irn Bru crate", 100-day tour of Scotland, when he took on all-comers, made him stand out. But what he describes as "continual campaigning" in his constituency over years has led him to increase his majority.
- Robert Halfon, the Conservative in Harlow, whose connection with his constituents means he's used as a barometer by other Conservatives
- Tim Farron from the Liberal Democrats, described by a source as "astonishing" on the ground.
What do they have in common? They have the argument, for good or ill, with the public, the people they represent. There are others of course.
The truth is, the overall outcome of the general election may not be much altered by the advance of the parties outside the familiar trio.
As the day nears, UKIP flirters may cleave to the Conservatives, or those tempted by the SNP may return to Labour.
But who wants to win by default?
Surely the most basic and vital act of political discourse ought to be part of the fraught search for answers to the worries about the fracturing of the political system.
Do you think most voters really want to be ignored or taken for granted, discuss the vagaries of Westminster, or be interviewed by pollsters with their ever-more sophisticated technology?
Or do they want a politician who hopes to represent them to knock on their door, ask what they want and listen to their response?
As long as they don't tell poor "knock-knock" gags, I think maybe we all know the answer.
If neither the Conservatives, nor Labour, nor the Liberal Democrats are prepared to go out in larger number to make their case and listen to voters' arguments, the joke could be on all of them.