Despite its importance, most modern people know little about fire. There are very few pure substances in nature. Dirt, rocks, sticks, bones, and even air are complex mixtures. The first distinction to be made is between heterogeneous and homogeneous substances. Almost everything in nature is heterogeneous , that is, its composition is not uniform.
Wood is a good example.
If you look at a cross section of a piece of wood, the rings are immediately apparent. These rings are alternating bands of light and dark colored wood which were laid down as the tree grew. This variation in the composition of the wood from one place in the sample to another is what makes it heterogeneous. Wood, then, is heterogeous through and through as is typical for natural substances. A few natural substances are homogeneous , that is, their composition is uniform.