The prudent sit on the sidelines. Optimists recognise the froth, but hope to get out in time by selling to a greater fool before the bubble pops. Those greater fools insist it is different this time and high valuations are justified by great opportunities.

Each approach faces serious problems — problems that buyers of Chinese shares are having to face as the excess in domestic stock markets spills over into Hong Kong.

Sitting on the sidelines sounds prudent when a third of Shanghai stocks with earnings estimates, and half of Shenzhen’s, trade at above 50 times forward earnings estimates. One clear sign of fizz: of 1,541 stocks trading in Shenzhen, 270 have at least doubled in price this year, and another 662 rose more than 50 per cent. Only three are down.