When something burns, it undergoes a chemical reaction with oxygen called combustion. During combustion, the fuel (the material that is burning) reacts with oxygen to produce heat and light. However, not all of the fuel is completely consumed in the reaction. Some of the fuel molecules break down into smaller molecules, such as carbon dioxide, water vapor, and smoke.

Smoke is a mixture of these smaller molecules, along with other particles such as ash and soot. The color of the smoke depends on the type of fuel that is burning. For example, wood smoke is typically white or gray, while oil smoke is black.

Smoke is also produced when there is not enough oxygen available for complete combustion. This can happen in a closed space, such as a room or a fireplace, or when the fuel is too wet. When there is not enough oxygen, the fuel does not burn completely and more smoke is produced.

Smoke can be harmful to human health, especially if it is inhaled. Smoke contains a number of pollutants, such as carbon monoxide, nitrogen dioxide, and particulate matter. These pollutants can irritate the eyes, nose, and throat, and they can also cause respiratory problems such as asthma and bronchitis. In addition, smoke can contain cancer-causing chemicals.