Question

In autumn, people pile fallen leaves and put them in a compost pile. After several weeks, the pile becomes warm. Where does the heat come from?

Answer 1

 The heat comes from the bacteria consuming readily decompose materials and the oxidation process heats up the compost pile from ambient air temperature. Bacteria consume carbon as a source of energy (to keep on eating) and nitrogen to build protein in their bodies (so they can grow and reproduce).