If you've never witnessed a currency crisis, here's how they usually go, in six simple steps:
1. Your currency gradually creeps downwards. This can happen over a matter of months as your current account deficit - the country's ledger with the rest of the world - deteriorates.
2. Suddenly, overnight, investors panic. Their withdrawals of money from your country, until now a steady stream, become a flood.
3. You raise interest rates - one point, two points, and then more.
4. But rather than being reassured, investors sense panic. The capital outflows accelerate and your currency collapse continues.
5. Eventually, there is nothing for it. You yank up interest rates to eye-watering levels in the full knowledge that your country will be plunged into recession; that unemployment will skyrocket. You dig into your international reserves.
6. Finally, when those reserves run out, you call on your neighbours or the International Monetary Fund for a bailout.
That's the pattern that has repeated itself on countless occasions in economic history, whether in Britain in 1992, in Asia in the late 1990s or in much of the Western world in the 1930s.
But, if you'd rather see a currency crisis in the flesh, look no further than Russia.
The country is sitting at stage four in this depressing sequence.
It is reliant on oil for around two thirds of its exports, so the recent fall in crude prices has been disastrous for an economy still reeling from the sanctions associated with its Crimea exploits around Ukraine.
On Monday, the rouble collapsed by a tenth. Then, in the middle of the night (1am, to be precise), the central bank raised interest rates from 10.5% to 17% (they were below 6% at the start of the year).
It was the biggest rate hike since 1998 (when rates were lifted to 150%) and it briefly stemmed the outflows.
But as the day went on the currency started to fall again, from 60 to the dollar to 70 to the dollar and, briefly, to almost 80 to the dollar.
It steadied up around lunchtime, though where it ends up the day is, frankly, anyone's guess.
That Russia is in big economic trouble is hardly news. It has been struggling for some months.
But as stage two above underlines, the shift from chronic underperformance to sudden crisis is usually very sudden and unpredictable. We are now past that.
Whether Russia can arrest things now or will succumb to stages five and six remains to be seen.
The country has sizeable foreign reserves thanks to many years of high oil prices.
It has shown repeatedly (think the 1990s, think Stalingrad and Leningrad) that its people can withstand deep economic hardship. But such reserves of money and fortitude can only last so long.
If oil prices remain low and investors continue to flee the country, it is not inconceivable that it eventually calls on its allies for some sort of financial assistance.
The big question then is whether the IMF (largely controlled by the US) would actually help out.
After all, it generally prefers not to lend to countries which are under international sanctions.
Winters are always hard in Russia. This year's will be one of the hardest yet.
