Who knows what this expression means?
　　Big Apple doesn't mean a "big apple" but a large city,namely,New York City of the USA.
　　However,Big Apple is only another name of New York,then how is Big Apple is connected to New York?
　　It is said that over 50 years ago agroup of American jaz players often toured all parts of country to make money.And they painted all the towns as apples growing on the apple tree.New York was the city where they made the most money among the cities when they performanced there.Now that other cities are called apples,New York is reasonablly named Big Apple.
　　Over a decade ago,the officials took "the Big Apple" as the symbol of their city in order th attract visitors to their city.After that,more and more visitors are attracted to New York for a tour and a visit.As a rsult,"the Big Apple" is used as another name of New York and has been spreaded all over the world.
　　人工翻译,如有不当,请指正并商榷.