Why does firewood crackle when it is thrown in a fire?
If you have ever "distilled wood", you already know the answer to this question.
Solids and liquids do not burn. First they vaporize to a gas, then the gas burns. When wood is burned, the wood itself vaporizes. Much of this vapor is gaseous wood alcohol and water/steam. If too much vapor is trapped inside the wood, it will exert pressure and cause the wood to pop which causes the crackle.
"Distilling Wood" can be done by heating wood chips in a test tube and condensing the effluent vapor. This produces water, wood alcohol and activated charcoal.
Christopher Ingram, B.S., Independent thinker
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'