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CHICAGO, IL - MARCH 17: Members of the plumbers' union dye the Chicago River green for St. Patrick's Day on March 17, 2012 in Chicago, Illinois. The River was first dyed green in 1962 and has become a St. Patrick's Day tradition in Chicago. (Photo by Brian Kersey/Getty Images)

Every year, the city of Chicago famously turns the Chicago River green for St. Patrick’s Day. But how do they do it?

The Chicago Tribune reports that the work is done the Saturday before St. Patrick’s Day, when the parade is held (or on Saturday in years that it falls on a Saturday).  The crew arrives early in the morning at the North Branch of the Chicago River, and use two motorboats, which are donated by volunteers – the larger boat usually holds four people and the smaller one holds two. The large boat starts dyeing the river at about 9 am. One person drives the boat and the other three use flour sifters to dump about 40 pounds of environmentally safe orange powder into the river. The powder turns green when it hits the water. ,The formula for the powder is top-secret, according to the Tribune, but it is believed to be vegetable-based, according to Time Out.

In 2003 one of the parade organizers told a reporter that revealing the formula would be like “telling where the leprechaun hides its gold,” according to Mental Floss.

The smaller boat follows the larger one and churns up the river to disperse the powder. It takes about 45 minutes for the river to turn completely green and depending on wind conditions, it could remain green for several days.

Other cities that dye their rivers green include San Antonio, Texas and Tampa, Florida.

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