The easiest way to make a balloon fly is to fill it with gases that are less dense than air, such as helium or nitrogen, but hydrogen is not recommended. Hydrogen is chemically active and prone to accidents, and helium is recommended to be safer.
The reason why the balloon flies is that the gas inside the balloon is less dense than air, and the gas exhaled by our population contains more carbon dioxide, which is more dense than air, so the balloon blown by the mouth must be sinking, and the balloon played with the ordinary air cylinder may float gently, may fall, but will not fly, because the gas inside the balloon and the air are the same.
Therefore, the easiest way to make a balloon fly is to fill it with a gas that is less dense than air, such as hydrogen or nitrogen.
Hydrogen is chemically active, prone to accidents, not very safe, not recommended to use, or recommend that you use helium is better. Helium is an inert gas, less dense than air, chemically stable, usually does not combine with other elements or compounds, it is not active, can not burn, and does not support combustion.