You ask a good and important question. How fast a fire can spread depends on many things, and I bet you can make a list of some of those things. Let's think about it. First, let's think about the stuff that is going to burn. If there is a lot of brush and dead trees that have fallen on the ground that gives the fire a lot of fuel - and makes it hot. If someone makes a fire in a fireplace, it goes out once the wood (fuel) is gone, right? You have to keep giving it wood to burn. So in a forest, having lots of fuel can allow the fire to spread fast. And if it gets hot enough and big enough, it can even catch the big, living trees on fire. Then it can really spread because it jumps from tree to tree. But what if it has been raining? Then everything gets wet, and the fire cannot burn that stuff very well. If you pour water on your campfire wood, then try to light a fire, it does not work very well, right? So having rain (or even high humidity - moisture in the air) can slow down a fire (and even put it out). Hmmm...What else might affect how a fire acts? How about wind? If it is windy, the fire gets lots of oxygen (which it needs to burn), plus, sparks get blown around and new fires can start, even a long ways away. So, if it's hot AND windy, fires can spread really, really fast.

Maybe you saw or heard about the Zaca Fire and how it burned much of the Los Padres forest. One reason it spread so fast was that there was lot of brush and dead plants for it to burn up AND it was really hot and dry. The firefighters build "breaks" to contain the fire - these breaks are just areas where the firefighters clear out the plants before the fire gets there - then there is nothing for it to burn (no fuel) and they hope it goes out.

I hope this answers some of your questions. Maybe you can your classmates can think of even more things that might affect a fire!