Why do some materials create fireballs when blown into flames?

[ahhppull]

Homework Statement

Cornstarch does not burn when its grouped together, but when you blow it into a flame, it creates a fireball. Why is this?

Homework Equations

n/a

The Attempt at a Solution

I found out that other materials, like flour and some metals, create a fireball or flame when you blow it into flames. I still can't figure out why this happens...

 

[Mark44]

Many materials are like this. For instance, it's very difficult to ignite wood in the form of a log using just a match, but if you cut the wood into small pieces, they catch fire very easily.

Can you think of a reason why cornstarch or flour or sawdust would be very flammable, but piles of cornstarch or flour and large pieces of would aren't as flammable?