Human respiration produces carbon dioxide.
Put a straw into the neck of each balloon and secure with masking tape. Inflate a balloon by blowing through the straw and pinch the balloon closed at the neck. Use the bicycle pump to inflate a second balloon of a different color and pinch the balloon closed at the neck. Release the amount of air to make it the same size as the first balloon and then seal with a twist tie. Repeat steps 6 and 7 with the second balloon.