When you blow a balloon with your mouth, you are filling it with air. Air is a mixture of different gases, including nitrogen, oxygen, and carbon dioxide. The main component of air is nitrogen, which makes up about 78% of its volume. Oxygen is the second-most abundant gas in air, making up about 21% of its volume. Carbon dioxide is a trace gas in air, making up only about 0.04% of its volume.

When you breathe out, you are expelling air from your lungs. This air contains a higher concentration of carbon dioxide than the air you inhaled, because your body produces carbon dioxide as a waste product of metabolism. When you blow this air into a balloon, you are transferring the carbon dioxide from your lungs into the balloon.

As a result, the air inside a balloon blown by mouth contains a higher concentration of carbon dioxide than the air outside the balloon. This can make the balloon feel slightly warm to the touch, because carbon dioxide is a greenhouse gas that traps heat.