This explanation of the existance of trees from one of Feynman's lectures is one my favorites. Atoms in all of the universe like each other in different degrees. Oxygen for instance, in the air would like to be next to carbon, and if they get close to each other they stick together. And if they’re not too close, they repel and they go apart.
If you set something like wood in oxygen, there’s carbon in wood from a tree and the oxygen comes and collides with the carbon, but not hard enough and it goes away again.if somehow they are made to go faster over the carbon they snap in and that produces a lot of rapid motion amongst atoms of carbon and oxygen, which certainly hit other atoms and make them got faster and all this motion produces heat, it leads to a disaster with all these atoms going faster and faster, that disaster is a fire.
Now, this terrible snapping is producing a lot of rapid motion amongst atoms which is basically producing fire.
Now if we go even deeper and wonder, how did this wood get all its carbon with the oxygen around it all this time?
Well it came from a tree and the substance of a tree is carbon,which comes from the air.
Most of us think that trees come from the ground,but if you actually look into it and find where the substance of the trees come from, they come from the air. Trees come out of the air. The carbon-DI-oxide
in the air goes into the tree and then the tree changes it by removing the oxygen and pushing the oxygen away from the carbon, therefore leaving carbon with water. Water comes out from the ground,but water got into the ground out of the air in the form of rain.So in fact almost all of the trees come out of the air.
The strong bonds between oxygen and carbon are broken by the tree, it does this with the help of sunlight. The sunlight comes down and pushes these oxygen away from the carbon and hence sunlight is essential for a plant to work. And the residue oxygen is released out of the plant or tree, which we all breath.As a result the carbon and water make the substance of the tree. Now when this wood is brought out to a fireplace, the carbon in the wood prefers to stick back to the oxygen and when they get back to the same activity as discussed in para 1, the heat that is coming out is the light and heat of the sun that went in and was stored. And is basically the sun which is coming out when you burn the wood.