Finer bits of wood, like sawdust, or pencil shavings from sharpening, catch fire much more readily than a solid chunk of wood like a whole pencil.

Given the right environment, finer sawdust can even be explosive.

A lot of campers and other outdoorsy types are probably familiar with using "feather sticks" to start a fire, where you take a stick and cut a bunch of fine curls into it, almost like you're whittling down the stick but leaving the shavings attached.

The whole stick wouldn't readily catch fire, but those finer curls attached to it will light pretty easily and spread to the rest of the stick.

And while I've seen some pretty impressive feather sticks made by people with a steady hand and sharp knife, most of the time those feathers aren't quite as fine as most pencil shavings.