Why do burning things make smoke?
During a fire, the air around the fire becomes heated. The heated air sweeps up water vapor (molecules of water that float in the air) and tiny specks of the fuel (the material being burned) into a dark cloud of smoke. The more incompletely something burns, the more smoke it produces, because more particles are left to be swept up into the air. Smoke gradually spreads out and drifts away, with gravity pulling the heaviest bits back to the ground. When a fire first starts to burn, there is usually a lot of smoke, which decreases as more of the fuel is burned completely. Smoke detectors take advantage of the fact that fires cause a lot of smoke in their early stages. The detectors sense the small particles in smoke before a fire really starts to burn. An optical smoke detector uses a light beam and light sensor that sounds an alarm when smoke particles get in the way of the beam. An ionizing smoke detector can sense even smaller particles; they disturb a low electric current inside, which sets off an alarm.