What will you observe when you place a small animal such as a live cockroach in one flask (A) and a dead cockroach in the other flask (B) where both flasks are fitted with a rubber cork which left for few hours, after that introduced with small burning candle into each flask and mouths are closed immediately as shown in the figure.
The flame goes off faster in the flask (A).
The flame goes off faster in the flask (A) containing the living animal, showing that some oxygen has been used up by the live cockroach during respiration.