Don't work with children or tapeworms
Never work, W.C. Fields supposedly said, with children or animals.
The same Fields was a sage chap. He it was who professed a liking for cooking with wine, adding: "Sometimes I put it in the food."
Politicians on the campaign trail, though, can scarcely follow the guidance re avoiding children.
Would not look good if a proud parent pressed his offspring upon a candidate only to be rebuffed with a snarl or a sniff. But how about animals?
Can one draw the line in the pursuit of votes? Can one say nyet to a pet?
Alex Salmond drew a bestial distinction today. Mind you, the creatures he shunned were tiny, wriggling and hideous. In short, dear reader, tapeworms.
How can this be? Has it come to this - that our first minister is to be pestered with parasites in the pursuit of Scotland's interests?
Mr Salmond was launching the SNP's European Elections campaign at Moredun Research Institute near Edinburgh.
It has a global reputation for scientific work on animal welfare.
Why there? Because it receives funding from the Scottish government and the EU - and thus can be depicted as an example of close co-operation between Edinburgh and Brussels.
Alex Salmond, Nicola Sturgeon and the six SNP candidates duly donned white coats for a tour.
(Was I the only observer who instantly thought of John Major's remark that every time he thought of a particular Eurosceptic colleague he "could hear the flapping of white coats." Quite probably I was.)
Anyway, around they went, deftly avoiding the sign which said "eyewash".
No, it was not a comment upon contemporary political discourse. It was a bottle of stuff for washing your eyes.
All was going well. Moredun is genuinely impressive: a scientific front line against infections and diseases which can blight animals and thus damage farm incomes.
(After nearly a century, it is still owned by the farming community.) The staff were charmingly enthusiastic, explaining the impact of their efforts in Scotland and around the world.
Then came the lab billed "Parasitology". The guests were invited to peer into a microscope to view the Barber's Pole tapeworm, a crowlin' ferlie which does endless harm to poor, defenceless sheep, particularly in Australia.
Defenceless, that is, until Moredun got on the case.
Sundry candidates had a glance, albeit fairly briefly. Nicola Sturgeon peered into the eye-pieces, seemingly undeterred.
By contrast, Alex Salmond was nominated but declined.
Perhaps he wondered what use the wicked media would make of the pictures. Perhaps, as he suggested, he is queasy about things which crawl - particularly when their destination of choice is the intestinal tract.
He didn't miss much. As a youngster, I possessed a microscope and showed, for a brief spell, an interest in the world of tiny, wriggling things.
And so I was ready for Moredun's mini display.
Perhaps their mothers love them. (Actually, tapeworms have both male and female reproductive systems . . . but you get the concept.)
To this observer, they looked rather the life which they inflict upon sheep - nasty, brutish and short.
In any case, the FM's focus was firmly upon the European Elections and the prospects for his party. Fairly bright, to heed Mr Salmond.
They topped the poll in Scotland last time around - and hope to replicate that, perhaps adding another seat to the two which they already hold.
Their pitch will be that they are the only party which stands up, ineluctably, for Scottish interests.
As ever, of course, there is a broader objective. As ever, that is the independence referendum. Mr Salmond hopes for an advance on two fronts.
One, he believes that the European Elections - in addition to the primary purpose of choosing MEPs - will spotlight the issue of Scotland in Europe and thus, perhaps, add a galvanic jolt to the already febrile referendum campaign.
Secondly, he muses the European Elections may result in a disparate approach north and south of the Border: that UKIP may triumph in England while the SNP head the polls in Scotland, that it will then be feasible to argue once again that politics in Scotland and England are diverging and require distinct treatment.
In short, independence.
Naturally, Mr Salmond's opponents will be out to thwart him in each of these objectives.
These European Elections may repay inspection for a range of reasons. Including the fact that the Brussels/Strasbourg Parliament has serious clout.
Towards the end of the launch this morning, the first minister approached the press pack - who were standing ready to ask him once more about his comments anent President Putin.
Perhaps seeking to pre-empt this, the FM walked up with a broad grin and announced that he had found a job for each and every correspondent at Holyrood?
And what might that be? Testers. At Moredun.