Huff and puff apparatus
The Wikibook Transwiki has a page on the topic of: Huff and puff apparatus
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 lime water or bicarbonate indicator shows the increased presence of carbon dioxide in the outgoing breath.This turns the bicarbonate into milky white substance.