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As indicated by the questions on the top part of page 5 of the Student Handout, breathing into the bag results in increased concentration of CO2 in the bag and decreased concentration of O2. This in turn results in increased concentrations of CO2 and decreased concentration of O2 in the blood and in the brain. The length of time you can hold your breath decreases as a result of these changes. If you carry out the Additional Activity described on page 2 of the Teacher Preparation Notes, the results will demonstrate that the brain is responding specifically to changes in CO2 levels, not O2 levels. In fact, it is the increased concentration of H+ (which is generated when CO2 combines with H2O in the blood to form carbonic acid) which stimulates the brain to resume or increase breathing.