Chinese state media says the country has spent much more on the event than was spent on the Beijing Olympics.

At least 70 million visitors - almost all of them Chinese - are expected to visit pavilions and exhibitions staged by more than 240 countries and organisations from around the world.

But it is hard to say for certain these days what a World Expo is for.

In the past they were used to promote a country's strength as a world power.

The first one - the Great Exhibition at Crystal Palace in London in 1851 - was designed to demonstrate Britain's industrial, military and economic superiority.

To China, this year's Shanghai World Expo is no less important.

It is an opportunity, officials say, to promote China's "soft power" - to show off its growing influence in the world - and, of course, for others to try to win favour with the Chinese.