World expos have a 159-year history.In concept, their main forerunners were the fairs put on by European merchants at regularly scheduled times during the middle ages. The first expo was held in London in 1851. It was the era when the sun never set on the British Empire, and in order to show off the fruits of the first Industrial Revolution, which had occurred first in Britain, the British organized a "Great Exhibition." Queen Victoria extended invitations to 28 other nations around the world. Among the objects on display were steam engines, telegraphs, industrial looms, and other important inventions and devices connected to the Industrial Revolution, such as metallurgic and agricultural machinery.

Although airplanes didn't exist back then, some 6.3 million visitors attended the exhibition during its 140-day run. It was an epochal event, when British imperial might was at its peak, and the exhibition helped to hasten the Industrial Revolution in other countries and inspire other powers in Europe and the Americas to hold exhibitions of their own.

Promoting exchange

Those early expos were largely mass-market general fairs, and there was nothing regular about their scheduling. Typically, they were held on the anniversary of some important international event. For instance, the Centennial International Exhibition, where Alexander Graham Bell's telephone was first demonstrated, was held in Philadelphia in 1876 in commemoration of the 100th anniversary of American independence. Paris built the 324-meter-high Eiffel Tower for its 1889 Exposition Universelle, which commemorated the 100th anniversary of the French Revolution. The tower heralded the age of steel-frame construction. The 1893 Chicago World's Fair marked the 400th anniversary of Columbus discovering the New World. Its Midway Plaisance featured amusement-park-type rides (a first at a World Expo), including the world's first Ferris wheel.

The model for these exhibitions began to change in 1933, when America and Europe were struggling to pull out of the Great Depression. Chicago held a world's fair named the "Century of Progress Exhibition" that documented technological inventions and innovations, portraying them as a prime force behind human advance and social development. The exhibition also demonstrated a transportation system that would have a deep and lasting impact in the years to come: the freeway or high-speed motorway.

Ever since, each hosting nation has picked a clear theme. These have included such broad topics as energy, transportation, art, space, freshwater, the oceans, and the residential environment. World Expos thus evolved from "non-sales product fairs" or "achievement exhibitions" to "thematic expos" with exhibits revolving around one main theme.

Interestingly, the main themes often reflect what's in the "atmosphere" of global society at that moment. For instance, between the two world wars and then later during the Cold War, World Fairs often revolved around themes such as "peace" and "building a better tomorrow." For instance, the slogan of the Brussels International Exposition of 1935 was "Peace through Competition," and the slogan of the 1939-40 New York World Fair was "Dawn of a New Day." That of Japan's Expo '70, meanwhile, was "Progress and Harmony for Mankind."

Since the end of the 20th century, people have been showing great concern about environmental themes such as natural conservation and sustainability, and these have repeatedly found their way into World Expos. For instance, Expo 2000 in Hanover, Germany chose "Humankind-Nature-Technology" for its theme, and Expo 2005 in Aichi, Japan, adopted "Nature's Wisdom."

Shanghai picked its theme for Expo 2010-"Better City, Better Life"-in the hope that urban development experiences from cities around the world would help to provide solutions for problems plaguing cities such as "extremes of wealth and poverty," "scarcity of economic resources," "environmental pollution," and "cultural conflict."

What's next? For Expo 2015, Milan has chosen the theme: "Feeding the Planet, Energy for Life." It plans on analyzing the connection between culinary ingredients and cuisine in participating nations, examining the subject from many angles-including culture, tradition, technology, and creativity. It will also explore how to ensure-in a safe manner, while protecting both the environment and the public interest-that there will be enough food to go around in spite of gradually depleting agricultural resources.