Presenter notes: Open by having the whole class hold their breath while you time it. Forty seconds is typical and somebody will manage a minute. Then ask the question that matters: what made you stop? Everybody says they ran out of air. Tell them that is wrong and do not explain yet. There was still oxygen in their blood when they gave up. Something else forced them to breathe, and by the end of the period they will know what it was and why it proves that breathing and respiration are two different things.