Most people recognize the name Erin Brockovich from the 2000 movie that starred Julia Roberts. The real life Erin Brockovich, an environmental activist, stopped by "The Late Show" and explained the Flint water crisis to host Stephen Colbert.

"The city was having some obvious financial issues, so they hired an emergency city manager, and they made a decision to switch the city water supply," Brockovich told Colbert. "Everybody's water quallity is different, so you can't just willy-nilly switch water systems. So they switched from the Detroit water, which is from Lake Huron, to the Flint River water, which is very corrosive. What happened was, when it hit the system, it caused all the lead, the iron, the maganese, and the copper to leach out from the pipes and all that lead was delivered to residents in their tap."

Brockovich explained that she had been involved in the water crisis in Flint before it even became national news.

"About a year ago they contacted me," Brockovich said. "People were concerned about the changes in color of the water, they were concerned about the smell of the water. They became concerned about their families, so oftentimes when we get called we'll send investigators out, which we did. And we even wrote a protocol for the city about how to treat the situation, so it didn't have such a fallout, this time last year."

Check out her fully explain the crisis in Flint below: