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The huff and puff apparatus is used in school biology labs to demonstrate that carbon dioxide is a product of respiration. A pupil breathes in and out of the middle tube. The glass tubing is arranged in such a way that one flask bubbles as the pupils breathes in, the other as the pupil breathes out. A suitable carbon dioxide indicator, such as limewater or bicarbonate indicator shows the increased presence of carbon dioxide in the outgoing breath. This turns the limewater into milky white substance.