With varying degrees of success, the Prime Minister tries to live by the edict "explaining is losing".
This does not apply, however, to explaining Labour's policies for them.
After David Shearer went all Whirling Dervish trying to explain the figures for his affordable housing policy, Key generously added his explanation adding a further word to the Prime Ministerial lexicon in the process.
"[They] promise people you will get a house for under $300,000. But when they wake up - BOOMFA! - it's a one bedroom bedsit in the CBD."
Shearer, meanwhile, was caught out having to explain too much to compensate for not explaining enough when the policy was first announced. It didn't help that his explanation was different every day.
That policy was initially set out simply at the party's annual conference last year.
It was to build 100,000 "affordable" houses over 10 years. For the first five years, two thirds would be in Auckland. And Labour could do it for less than $300,000 a house.
The policy was lauded far and wide. Labour was here, and would take New Zealand families to the promised land for a bargain price. It was manna for the renting middle class. The polls showed those families wanted to believe. In a Herald DigiPoll survey, nearly three-quarters of New Zealanders said they approved, including a sizeable portion of National Party voters. Then came the "buts". The $300,000 figure became an "average" rather than a maximum. Last Sunday, Shearer explained only little boxes in Auckland would be built for that sum - while larger family homes would cost more, although still under the average of $550,000 for a four-bedroom home.
Having scoffed at National's idea of $485,000 "affordable homes" at Hobsonville as unaffordable, Labour's own figures were getting perilously close to the same price. Little boxes - also known as apartments or "compact" terraced housing - would make up the majority of the homes under the scheme.
That's just lovely, but the trouble was he had pitched the policy as for "Kiwi families" - not Kiwi singles or even Kiwi couples. Kiwi families has a nice ring to it. But not many Kiwi families are comfortable living in a one-bedroom apartment. That's what Labour used to call "over-crowding".
On Monday, he'd reverted to talking about three-bedroom homes for $300,000 again. By Tuesday he'd shifted again and knocked $200,000 off his 4-bedroom homes, promising they would be "in the $300,000s."
And the Promised Land became Mangere, and the home a pre-fab.
Like Tom Hanks clinging to his life raft in Castaway, helplessly watching as Wilson floated away, Shearer could only watch as his policy began to attain the tarnish of reality. The emperor's threads were getting threadbare.
Shearer continued to try to insist Santa was real, saying they would be cheaper than the market price. That came from assurances by unnamed construction companies that it would be "substantially" cheaper because they could buy materials in bulk, although it was not clear exactly how much cheaper it would be.
Those construction companies remained unnamed, he said, because they did not want to get involved in the politics of it all. That is, of course, code for they didn't want to be blackballed from any lucrative contracts the National Government might toss their way.
To sum up what we've learned so far, some of Labour's homes will be affordable. Those will either be small units, or homes in suburbs that are already pretty affordable because nobody wants to live in them. Depending on your definition of "affordable", others will not be affordable, but will be cheaper than other unaffordable homes.
Still good, but not as good as it was originally touted back in November last year when Shearer had promised to deliver on the Kiwi dream by producing "entry-level houses that Kiwis are crying out for".
Of all the cries for affordable houses, none held the words "prefab in Mangere".
Much is made of the fact that previous generations setting up their first household had not been picky, and had settled for homes in the city's outer suburbs. But those outer suburbs were a lot closer to the city than today's outer suburbs. They are now inner suburbs.
Home ownership is one of the main triggers for middle-class angst and Labour's policy was pitched at that middle New Zealand "dream".
Instead, it is becomingly increasingly clear that it will fall short of what middle New Zealand wants for itself. The benefit is for those on lower incomes. There is nothing wrong with that, but it should have been clear from the start.
The reason it wasn't is because Labour will be hoping voters have only taken in what they wanted to hear, the big bang of the initial announcement, and have ignored the subsequent unpicking of the details over a hazy summer. But so arises another edict for politicians to live by: that gaping chasm between over-promising and under-delivering. Boomfa indeed.