#every three times i clean my bedroomy my mother makes me an apple pie,
cleaned my bedroom 9 times, how many apple pies does she owe me?
(what? your mother doesn't reward you for cleaning your bedroom? aren't
there child labor laws? to make up for that injustice, you may have this very
easy extra credit problem.)

You exhale co2 which is produced during cellular respiration.  co2 combines with the water in your blood's plasma to make up one half of the body's most important buffer pair, carbonic acid.  the more physical activity you engage in, the more co2 your body is producing.  you can see this by putting some of the cabbage indicator in a glass and then blowing bubbles into it through a straw.  can you see a change in the color of the indicator?