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 and density of smoke depends on the type of fuel that is burning. For example, wood smoke is typically white or gray, while gasoline smoke is black.

Smoke can be harmful to human health, as it contains many toxic chemicals. Inhaling smoke can cause respiratory problems, eye irritation, and even cancer.