At Phoenix-Mesa Gateway Airport, a plane loads up with fire retardant, that red stuff you see dropped on wildfires. 

Right now, the Cellar Fire near Prescott is one of the most critical in the country, and that's why the DC-10 and a smaller air tanker were at the airport loading up Wednesday afternoon.

RELATED: Lightning-caused wildfire burning south of Prescott grows to 7,000 acres

When the tankers land, crews reload the plane, refuel if they need to and get back to fighting the fire. 

Here's what you need to know about the fire retardant: First, don't call it slurry. Second, it's not harmful.

"I don't think anybody should go eating it or drinking it or licking it off the plants, but overall, it's not toxic to folks. People can handle it here on the base," said Rocky Gilbert with the U.S. Forest Service. 

There's some debate about what happens to the environment if fire crews use a lot of it, like in California. But it's also better than fire ripping through the land.

The Forest Service also has designated places around the country where they're not allowed to use it—usually streams and rivers.

The retardant is basically fertilizer. It contains ammonium phosphate salts that make wood char like charcoal instead of burning like a campfire. 

It doesn't actually put out a fire. It makes it less intense, giving fire crews a chance to fight the fire on the ground. 

And that red color doesn't actually do anything. It's just so other pilots can see where it's been dropped. 

"The coloring will stay out there in an environment like this in the desert where it's really dry. It can be out there for over a year," Gilbert said. 

That doesn't mean it's still working. The retardant only lasts for a few months. 

But the color tends to stick around until it's washed off. It also has a tendency to stain. The tarmac in Mesa is red pretty much all the time.