What goes down...
Why is the pound rising? Because it went down before. Why might it go down in the future? Because it is rising now.
You may find that a somewhat vacuous explanation for the recent strength of the pound - which hit a seven-month high against the dollar and the yen earlier today and has also gained ground on the euro. But there is an argument buried in the banality. Also some good news for the UK.
Think back to the last three months of 2008, when the pound was falling like a stone. At that time the fear was that the world financial system might be falling off a cliff, leading to Japanese-style deflation for much of the world and/or a repeat of the Great Depression.
If you're an international investor facing that kind of economic storm, the chances are you're going to be reluctant to store your money in the UK. As we now know, the growth of the City made us acutely vulnerable to a crisis in the banking system. And with our high levels of debt, the economy would also suffer more than most from a bout of falling prices - by raising the real value of that debt, it would make the burden of that debt even greater than it already is.
Against that backdrop, investors needed plenty of persuading to invest in the UK. In other words, sterling assets have to offer something extra in return. Typically that might come in the form of higher official interest rates. But if you remember, rates were being slashed at the end of last year as well.
The best hope for a decent return from sterling assets was a future rise in the value of the pound. And the best reason to expect the pound to rise was it had fallen so far it could surely only go up.
Welcome to the "overshooting" theory of exchange rates. (Or a version of it, anyway. Its author, Rudi Dornbusch, might not recognise it.) Currencies have to go down a lot for people to expect them to go up.
If you don't like that theory - I have others. Economists are fantastically good at explaining currency movements - they can give you at least four for any particular change. What they're bad at is predicting them. Academic studies have found that complex economic models for forecasting exchange rates are precisely as reliable as tossing a coin.
The basic point is that currency movements - like most things in the markets - tend to be more focused on where we are going than we are now.
When we were heading for financial armageddon, sterling looked a poor bet. Now things look calmer, the UK starts to look like a safer home for investors' money, for all the reasons it looked scary before.
That's especially true when data such as the today's Purchasing Managers' Index for the service sector - and manufacturing data earlier in the week - point to a slightly faster return to growth than previously thought.
If investors now think the recovery will come sooner - perhaps sooner than in continental Europe - they might also conclude that interest rates will rise more quickly as well. That is probably also raising the attractions of sterling right now.
One conclusion we can tentatively draw from Sterling's mini-surge. Investors right now are not much bothered about a return to high inflation in the UK. As Martin Wolf argues in his column today, the recent rise in government bond yields suggests that inflation expectations are going back to normal levels. But there is little sign that investors expect inflation to take off.
If and when investors do start to worry about excess inflation in the UK - driven, perhaps, by concerns about the level of public debt - the pound could fall again. It depends
on what they expect to happen to interest rates in response. But yes, how far it could fall in the future will also depend on how far it's risen today.