You got to see a plane dropping a fire retardant. Pretty cool. To retard something means to slow down or stop something. Fire retardants are used to smother fires. Fire needs three things: oxygen, fuel (like wood), and heat.

You can stop fires by removing any one of these three things.Firefighters can use water to cool a fire. The problem is that the water runs off the fuel and evaporates quickly in the heat of the fire.Fire retardants that they drop from planes contain things that make them sticky so that that they coat the fuel. This keeps the fuel cooler and away from the oxygen so that it doesn't burn well.

They color the fire retardant red so that the air crews can tell where they have already dropped the retardant.

Sometimes firefighters actually start fires in front of a fire. These are called "backfires." They only do this when they can control the backfires. When they are done, they have a big burned patch between the fire and the place they want to protect. Why do you think this works?

Fires aren't all bad. Some plants need fire in order to grow. Fires also keep dead wood from piling up and return nutrients to the soil.Once a forest or grassland has had a fire, it is usually a long time before it has another one. Can you figure out why?

The material is called Foscheck.It is primarily made up of 85% water, 10%fertilizer (ammonia phosphate and sulfate ions), and 5% minor ingredients(iron oxide for color, clay or bentonite). It works by coating the ground,buildings and plants with a moisture (like water, but has a 'stickyness' to it) usually dropped by aircraft and reducing the flammability. It is non-toxic to humans if it gets on your skin but can be harmful to animals if swallowed. It is red because once dropped it is easily visible from the air and on the ground.

They're dropping Phos-Chek.
click_here
The company that makes it has a video about it here:
click_here

I heard on the news right now that they're hoping the PhostrEx dropped by the DC-10 will keep the fire from moving over the ridge top, meaning it was used mostly as a preventative measure. Wikipedia lists the ways these types of fire retardants can extinguish a fire:

The plane was spreading fire retardant. Fire retardants work three ways: removing heat, coating the surface (so the fuel can't find oxygen), or by diluting the air near the fire.

Water is a great fire retardant because it does all three. It cools the surface while it evaporates, and the resulting steam also dilutes the air, pushing oxygen away. Unfortunately, water can also flow away, soak into the ground, or evaporate too quickly. So other materials are used, such as aluminum hydroxide or special, sticky gels. These materials serve the same purpose as water, but they don't evaporate or soak into the ground.

Many fire retardants are clear in color. Firefighters add a colored dye so they can clearly see which areas have already been sprayed. That's why the spray you saw was red. The color will usually fade in a few weeks. Fire retardants sprayed from aircraft usually don't extinguish fires. Instead they slow or cool a fire so firefighters on the ground can safely get to it and extinguish it or set up a fire break.