When bedrock is broken down into small rock pieces, soil starts to form. The breakdown is caused mostly by rain, ice, wind, freezing and thawing, or chemical changes. Plant roots spreading downward break these pieces apart further. Some burrowing animals make tunnels that fill with air and water. This water expands as it freezes, causing more breakdown. Dead animals and plants are decomposed by bacteria and fungi for energy. The leftover decomposed plants and animals, called humus, mix with the rock pieces forming soil.