Respiration by living organisms can be investigated by carrying out experiments to show the production of carbon dioxide.
If carbon dioxide is bubbled through limewater then it turns from clear to cloudy/milky in colour. This is why limewater used in a simple respirometer can show that more carbon dioxide is present in exhaled air compared to inhaled air.
Breathing in through the mouthpiece draws air from the atmosphere in through tube A. Breathing out through the mouthpiece bubbles exhaled air through tube B. After a few breaths the limewater in tube B will turn cloudy as the exhaled air contains more carbon dioxide.
If inhaled air contains 0.04% carbon dioxide and exhaled air 4%, how many times more carbon dioxide do you breathe out than you breathe in?
100 times more.