Blowing out birthday candles on a cake is an old tradition. But why does the candle go out when you blow on it? That happens for 3 different reasons. One is that your breath pushes away all of the Oxygen gas. Your breath is mostly made out of Carbon dioxide. The Carbon dioxide pushes away the Oxygen near the flame, which it needs to stay lit. Fire burns using the Oxygen gas. Carbon dioxide puts the flame out. Another reason is that the warm air around the fire gets blown away. Fire needs heat to burn and stay burning. The hot air around it helps fuel the fire. When it is blown away, the fire goes out. The last reason is only if the thing burning is candle wax. The air from your mouth cools down the wax. That causes the wax to solidify and stops the burning of the candle. The candle gets put out because one or more of the “sides” of the “Fire Triangle” are not in place.