Stop me if you have heard this one before. But the accidental spilling of more than HK$52 million from an armoured van on Gloucester Road on Christmas Eve by the under-alert guards of security firm G4S reminds me of an old joke about economics. It goes like this: An economist and a normal person are walking down the street together. The normal person says, "Hey, look, there's a HK$20 bill on the sidewalk!" Without looking, the economist completely dismisses it: "That's impossible - if there were really a HK$20 bill, it would have been picked up by now." As seen from news footage, there appear to be many normal people who risked life and limb and interrupted traffic to pick up as many freshly minted HK$500 bills as they could. Some news stories also hinted that several mainland tour buses passed by and some of the missing money might already have been taken across the border. Some editorials and headlines lament the greed and ugliness of human nature displayed in the incident. But economists would react differently. That joke is less about economics in general than about what's called the efficient market hypothesis, which dominated post-war economics and finance - until the global financial crisis shook faith in it. It mocks those economists who take the concept of market efficiency too far as to exclude the real-life possibility of free money. The idea is that consistent returns (free money) in financial markets are impossible because market prices - as tracked by millions of investors, analysts and market players who make up "the market" - reflect all available information. Now, if an economist can't believe in the possibility of HK$20 lying around, what would he or she make of HK$52.5 million spilled all over a busy street? After the Gloucester Road cash spill which grabbed world news headlines, the economist joke may soon have a new version: Buses and coaches are separately carrying groups of mainland tourists, locals and economists, the latter on their way to an academic conference. Greedy mainlanders and locals fight each other over the money. But the virtuous economists stay put and instruct their driver to move on because what is happening is impossible.