There is a rather clever way of comparing countries around the world called the Big Mac Index.

It’s all about the price of a Big Mac hamburger from McDonald’s, which is basically the same product wherever you are, and so once you’ve weighed in a few things – how much transport and wages cost from one country to another – you can find out where you get the best value.

It started out as a light-hearted idea in The Economist three decades ago.

Now people look at it closely, discuss it and, I should imagine, make some decisions based on its findings.

And if you’re travelling globally, and you’re partial to burgers, it’s probably a good guide to where you’re going to feel ripped off, and where you’ll digest your meal reckoning you got a bit of a bargain.

Retailers in every country know selling stuff is a balancing act between getting a good price, and so making a handy profit, but also sending the customer home happy.

A good deal, someone once told me, is one where both sides reckon they came out best.

Take the price of Haribo Tangfastics, for instance, a foodstuff with which I am slightly acquainted.

Buy them in Budgen’s in Chorleywood, and you’ll fork out just over £1. Poundland might sell them for a quid.

At a motorway service station, where you’re captive and helpless, they’ll try to charge you north of £2.

All for the same thing. You’d leave one store feeling you got a decent deal; on the motorway, you’d feel ripped off, even if you did give in to temptation.

So how to find the right price? How to take our money, but leave us happy?

Well, if you want the answer, head to a school fair.

Few places or events are so adept at separating you from your cash, but making you feel good about the process.

Nobody can begrudge putting a few quid in the kitty at the fair, and it’s only when you get home that you realise that “a few quid” turned out to be a lot, lot more.

The Christmas fair – please, never “fayre” – is a staple of our school, has been for years and will, no doubt, go on for all eternity.

And you know what? Long after I’m gone, I reckon the stalls will be the same.

Why? Well, because they work. Sure there are some changes round the edges, but the basic premise will be the same.

The ingredients that blend so well won’t change in 100 years’ time, even if we pay with our watch and we’re all wearing silver jumpsuits.

So there’s the tombola, for instance. What could be more simple, or more beguiling?

A drum full of tickets and the certain knowledge that all the winning tickets are in there somewhere.

I never win, but I’ll have a go in the hope of going home with a prize I don’t really want.

There’s the bottle tombola, where the person in front of me always wins.

I think it’s also traditional for a teacher to win a bottle of wine and for the nearest parent to say with a knowing wink “looking after this lot – I think you've earned that!”

And the toy stall – ah the toy stall. I’m on duty there tomorrow (Saturday) afternoon and, frankly, I’m expecting mayhem.

Or at least a brisk trade. I mean – kids, cheap toys, Christmas on the horizon? It’s the perfect storm.

Were it not for the proliferation of cuddly toys, I’d be very content.

But cuddly toys are an issue. You could insulate our loft with the cuddly toys that have come in and out of our family thanks to the cheap offerings at Christmas fairs.

They get briefly cuddled, then forgotten, then added to the bag of cast-offs in the attic.

And when the cast-offs get given to the Christmas fair, someone will try to buy them back.

It becomes a jousting match between my son and I. Whatever I do, he’ll somehow find a way to come home with an arm of cuddly bears, dogs, fish and whatever else.

That’s one of the reasons I’m on the toy stall this year – to monitor and intercept.

Hot food, of course, and mulled wine.

The raffle, which must – must – include some food in a wicker basket.

A cake stall (one day, I think there’s going to be a full-on riot in the race to be first to the cake selection), a book stall, which I will try, and fail, to resist, and probably face-painting and cake-decorating for children.

I love it. Love it all. It feels like the start of the Christmas season, a modern rite of passage to go alongside fusing the fairy lights, buying the Christmas Radio Times and giving half my earnings to Amazon.

Christmas gets going far too early these days, with mince pies on the shelves in September.

The Yuletide starting pistol actually goes this Saturday lunchtime.

Christmas fair means Christmas season is under way. Mulled wine? Don’t mind if I do...

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