Combustion
Combustion is a chemical reaction where a substance burns by combining with oxygen, releasing heat and light.
Think of it this way
When you light a candle, the wax is the fuel and the air around it holds the oxygen. The flame grabs oxygen from the air and reacts with the wax. This reaction releases heat and the bright light you see. Blow out the candle, and the reaction stops because the fuel and oxygen can no longer combine.
Explaining combustion by grade level
When you light a candle, the wax melts and travels up the string in the middle. At the top, the hot wax meets the air and catches fire. The flame gives off heat you can feel and light you can see. If you put a jar over the candle, the flame goes out because it runs out of air.