Why call big surprises black swan?

It goes back to the 2nd century Roman poet Juvenal, who is said that some events are "as rare a bird as a black swan." In realty, black swans aren't so rare but Europeans once believed there was no such thing because they ad never seen one. This certainty evaporated when early European explorers of Australia came upon swans whose feathers were black. The term has recently come into vogue thanks to Nassim Nicholas Taleb's book the The Black Swan : The Impact of the Highly Improbable, which became a surprise best seller after the 2008 financial crisis As defined by Taleb, a black rack swan is not just an unpresedented event but one with a large spread impact. "A small number of black swans," Taleb writes, "explain almost everything in our world. A symbol of rare, transforming occurences.------------------------------------------------------------------------------------------------------------------------------------------------------------------------Swans will keep the ponds and lakes cleaner
--------------------------------------------------------------------------the week april 8,2011