Candle Staircase

On every step of a small staircase is a candle. The staircase is contained on both sides by glass plates. When carbon dioxide is poured on the top step, the topmost candle is extinguished, followed by the one below it, and this continues until gradually all the candles down to the bottom are put out.

How does it work?
CO2 has a higher density than air and therefore sinks to the bottom. As it pushes the air out the way, the candles cannot continue to burn so go out one by one.