Once a year, in honor of St. Patrick's Day, the Chicago River in Chicago turns green.
The tradition began in 1962, when city plumbers, with the permission of the mayor, poured emerald green dye into the river.
The original idea came from plumbers who used special green dye to check for illegal sewage in the river.
In the 1960's, 100 pounds of dye were used to turn the river green for a whole week. Now it's only 40 pounds of vegetable dye, and the color lasts only for hours. Environmentalists feared that the use of the original dye would cause problems for wildlife in the river.
Today, the dye is a secret formula that is safe for all of the river's living organisms, including the kayakers and boaters who might fall in!
Other cities have followed Chicago's lead. The water in London's Trafalgar Square fountain has been dyed green. In 2009, First Lady Michelle Obama, a Chicago native, had the water in the White House fountains dyed green on St. Patrick's Day.
More people view the Chicago River on St. Patrick's Day than on any other day of the year.