Every fall homeowners have the same problem: dead leaves. Trees shed leaves and contaminate lawns once perfected. To get rid of these leaves, people rake them and assemble them into large piles that trucks come to remove. If these batteries are left long enough, they will overheat. On a cold day, you can see steam rising from these piles of leaves. But why does he do this? Decomposition is the breaking down of a substance into the parts of which it is composed. Microorganisms break down organic matter during composting and produce carbon dioxide, water, heat and humus. Composting usually consists of three phases: 1) the mesophilic phase (moderate temperature) which lasts two days, 2) the thermophilic phase (high temperature) which lasts four to six days and 3) the cooling and maturation phase, which it can last up to several months (Columbia University Press, 2000). Mesophilic microorganisms perform the first phase. Mesophilic microorganisms break down stable and easily degradable compounds. The heat they create causes the temperature of the compost to rise rapidly. The second phase (high temperatures) accelerates the breakdown of proteins, fats and complex carbohydrates, at temperatures of 55 degrees and above. Many microorganisms pathogenic to humans or plants are destroyed. Small holes are needed to ventilate this step. The third phase involves the supply of high-energy compounds that are depleted. The temperature of the compost gradually decreases, and mesophilic microorganisms take over in the final "curing" stage (maturation of the remaining organic matter (Southwestern, 2003).) There are three main gases in compost piles: carbon, nitrogen, and oxygen. Carbon provides the energy source and is the "building block" that makes up 50% of the mass of microbial cells. Brown and woody materials are very rich in carbon. Carbon is more readily available for microbial use if an object has a large surface area. Nitrogen is an important component of proteins, nucleic acid, amino acids, enzymes and coentins necessary for cell growth and function. All green, moist materials are rich in nitrogen. The nitrogen cycle is the continuous movement of nitrogen from the atmosphere to plants and back into the atmosphere (or directly into plants). Oxygen is essential for the functioning of a compost pile. As microorganisms oxidize carbon for energy, oxygen is consumed and carbon dioxide is produced. Without enough oxygen, the process will become anaerobic and produce bad odors. Oxygen concentrations above 10% are considered best for maintaining aerobic composting.