Experiment proving that exhaled air contains more carbon dioxide than inhaled air

(A, B and C refer to the tubes unless specified)

Close end of B. Breathe in. Air from atmosphere enters through A, passes through limewater and enters your mouth (cannot enter through B because closed).

Open end of B and close end of A. Breathe out. Exhaled air passes through limewater in test tube B and exits through B.

Limewater in test tube B (which contained the air you exhaled) turns milky much faster than in test tube A (which contained the air you inhaled).