Ice is placed in the bottom of a basin, and a glass watch glass placed on top of the ice. The temperature of the air in the room is determined and the air temperature just above the glass watch plate determined. A piece of string or rope is burned, the flame blown out and the smoking string placed on the glass. The smoke only rises until it hits the warmer air above the ice, then it stops. Normally, warmer air is closer to the surface than cooler air and the smoke would rise through it. This demonstrates now smog is produced in cities.