CHICAGO, IL (CNN) -- There's no mistaking which holiday Chicago is celebrating this weekend. Check out the Chicago River, which has been dyed green for St. Patrick's Day!
Tom Rowan has been making the river run green for the last 56 years.
Rowan, a 75-year-old retired police officer, is the man behind the famous Windy City ritual. The city's St. Patrick's Day parade is world-renowned, primarily because of its unique tradition of dyeing the river green.
"The original Mayor Daley in the 1950s wanted to do something special; he wanted to dye the whole of Lake Michigan green. But it was just too huge to do. The next best thing was the Chicago River," Rowan told CNN.
But the dye recipe is a closely guarded secret.
The dye itself is an orange powder, derived from an environmentally-friendly vegetable base.
But what exactly is the dye made of? Well, that's top secret.
"It's like the Coca Cola recipe -- we don't tell anyone," Rowan said.
Many other cities have tried and failed to replicate the dyeing process. One city official tried to talk the secret recipe out him recently, but Rowan said he held firm and disclosed nothing. "I wasn't born last night."
The orange powder only turns the shamrock shade of green once it hits the water. The magical transformation has led to it being called "Leprechaun Dust."
More than 40 pounds of the secret formula are plunged into the river. It is dispersed using a common kitchen implement: "A flour sifter is the tried and tested method," Rowand said.
Two boats are required for the dyeing process.
"One boat will do the dyeing and the other boat will do the mixing. You have to churn the water up."
The second boat is referred to as the "Mixmaster."
"We give the river a big dose and then we go up and see if we missed any spots," he said.
The methods have changed over the years.
"One of the ways my father tried was with huge fire extinguishers. The orange dye went all over the local buildings and people, but not one complaint did we get," Rowan recounted.
Tom Rowan has only broken from tradition once and dyed the river another color. To celebrate the Cubs winning the World Series in November 2016, he helped turn the river a brilliant blue in their honor.
Asked about the future, Rowan said that even after 54 years, he has no intention of retiring from dye duty. But, he noted, he is "one of nine Rowan children, so there will always be someone to fill that boat."
"It's a family thing. It's a tradition."