Why is there less Oxygen in exhaled air than in inhaled air? Oxygen in water etc.Please help a Yr 11 Schoolboy?

From what I can tell, it doesn't have anything to do with the amount of oxygen in the water, you blew into the tube in the water to fill the flask through water displacement, to just have your exhaled air in the tube, the water kinda keeps the other air out. It just means that your body (lungs) uses up some of the oxygen that is in the air and converts some of it to CO2 or carbon dioxide. Since the flame needs oxygen to burn, it can burn longer since the flask with the air from the room has more oxygen in it.