Some might not know the price of a pint of milk - but politicians just can't get enough of supermarkets. What's going on?

There's nothing fishy about this photo opportunity, except, perhaps, what the deputy prime minister was doing in a West London Tesco in the first place.

After all, Nick Clegg's speech on Wednesday had very little to do with business, or retail policy, or, indeed, fish. What message was he trying to send out?

"He is a Waitrose kind of guy who wants to be seen in Tesco," says brand marketing guru Chris Bullick.

"We like to see them out and about. It puts them more in touch in the real world," says Claire Beale, editor of Campaign magazine, who takes a slightly more charitable view of politicians, such as Tony Blair in this 2006 visit to a Marks and Spencer's store in London, mixing with the trolley-wielding classes.

There is also plenty of opportunity for purposeful pointing.

Or even, as in this picture from a 2010 visit to an Asda supermarket in Wakefield, a moment of sober reflection.

The trick is not give the photographers too much of an open goal. Try to steer clear of obvious caption fodder.

Make up your own porky pie jokes for this picture of the deputy prime minister in a Notting Hill Sainsbury's last year.

It also helps to have the right money ready - and avoid getting into arguments with the staff.

On this 1997 election campaign trip to a Tesco supermarket in Maldon, Essex, check-out worker Shirley Taylor refused to accept the former prime minister's cheque because she did not have a cheque guarantee card.

Unlike Lady Thatcher, who was actually buying something, today's politicians increasingly use supermarkets - and other places of work - as a backdrop for policy announcements and Q&A sessions.

Workers on a lunchtime break tend to be a lot better behaved - and easier to control - than random members of the public. Throw an egg, or an awkward question, and you are likely to lose your job.

For the politician, it is a chance to associate themselves with a solid, dependable household name.

"Politician's brand values go up and down like a yo-yo," says Chris Bullick, managing director of digital branding agency Pull.

"Nick Clegg is far more of a chameleon than any of the supermarket brands, there is no question about that."

For the supermarkets, it might just be a case of "any news is good news," he adds, a chance to get their name on the TV news bulletins.

But if a political leader is going to make the effort to break out of the stuffy confines of Westminster, they should at least try to have some fun with it.

It's not clear what Gordon Brown is about to tell Tesco workers in Newcastle, but judging from the look on wife Sarah's face - not to mention Lord Mandelson - it can't be good.

On the other hand, it is possible to have too much fun.

"He must stop waving his arms around," advised Guardian sketch writer Simon Hoggart, of this 2011 appearance by Ed Miliband at a vast Asda supermarket in Clapham Junction.

"If you do that in front of an audience of checkout personnel, they are going to wonder why you are bagging imaginary groceries."

And, finally, it is worth remembering that not every politician in a supermarket is there to press the flesh. In 2011, Alex Salmond was involved in a bizarre stand-off with Scottish Labour leader Ian Gray at an Asda store in Ardrossan.

The SNP leader (pictured here in a different store), had arrived by helicopter for a campaign visit. But aides reportedly flew into a panic when they spotted Mr Gray in the aisles and, fearing an ambush, began to steer their man away from the scene as quickly as possible.

But there was to be no ugly confrontation by the cooked meat counter. Mr Gray had simply popped in to the store "pick up some provisions", he explained, and had been unaware of his rival's presence.