You supply your body with oxygen by inhaling fresh air.  Your body cells need oxygen from the air to "burn" sugar, fat and other nutrients. After this chemical reaction you will exhale consumed air.
By the following experiment you will learn the difference between fresh air and consumed air.
Experiment:
* Left: Pour 20 ml of  lime water into two 50-ml infusion bottles.
* Close the right bottle by stopper pierced by two tubings: The longer one dips into lime water, the shorter one is for inhaling of air.
* Inhale by sucking one long breath of fresh air through the short green tubing.
* After that exhale this air  through a drinking straw into the lime water of the left bottle.
Observation:
* Right: The exhaled air makes lime water (left bottle) milky while inhaled does not change it (right bottle).
Explanation: Lime water is an indicator to test for carbon dioxide.