A funny thing happened earlier this week.

On the very day I dig out my trusty brown Birkenstocks, I learn that the clunky German sandals are bang on trend again. Must be my fashion sixth sense.

I also learn that the two-strap ones I have are called Arizona and are particularly sought after, so much so that they're flying off the shelves, according to online retailer Asos, who say sales of sandals are up by as much as 95 per cent.

"Demand is so huge this year that we can't meet it. We've got workers pulling all kinds of shifts. They've pulled out all the stops to meet demand," says Robert Lusk of the Natural Shoe Company, which sells Birkenstocks.

Prices are up, too. If you want to splash out on Givenchy's take on the Birkenstock sandal - they call theirs Swiss Sandals - they'll set you back £430. Even the leather version of the Birkenstock Arizona itself comes in at around £64 these days.

So how has this come about? Three reasons, two that are probably true, one that's so clearly ridiculous it's probably true as well.

First, fashion magazine editors realised Birkenstock sandals are actually quite comfy. Second, fashion designers started sending their models down the catwalk wearing them, possibly for the same reason. And three, something called Normcore happened.

Basically, this turns on the theory that Brooklyn hipsters rediscovered sandals when they started mimicking the dress sense of those tourists from less fashionable parts of America who come to New York to gawp at Brooklyn hipsters. (And they wonder why everyone hates them). Mind you, this presupposes that Birkenstocks never went out of fashion in places like Arizona which, though seductive as a theory given that the company went so far as to name a sandal after the state, seems unlikely. Like Brazilian footballer Neymar's haircut, we're just going to have to puzzle over that one.

The corollary of this, I suppose, is that the flip-flop has had its day as the summer footwear of choice for young people. This is a good thing. For a start, it's going to make running for buses (or trams, if you're an Edinburgher) easier. It also means our streets will be quieter because the Jack Wills brigade - those young men who insist on wearing chino shorts, T-shirts, slouchy woollen hats and flip-flops whatever the weather - will have to find a less noisy way to round off their look. No more dropping your latte in fright outside Urban Outfitters because you think you've heard pistol shots behind you. C'mon, we've all done it.

Of course, the news that Birkenstocks and Birkenstock-style sandals are back in fashion raises the question of whether socks should ever be teamed with sandals. It was never an issue when flip-flops ruled. Now it's back, and troubling us again. It needn't. The basic rule of thumb (or big toe, if you prefer) is this: if it's cold enough to need socks, it's not warm enough for sandals. That's just basic fashion maths.