The main gas in a balloon is typically helium or air. Helium is a lightweight, inert gas that is often used in balloons due to its lighter-than-air properties, allowing balloons to float. It is non-toxic and non-flammable, making it a safe choice for various balloon applications. Air, composed of nitrogen, oxygen, and other gases, is another common filling, especially in decorative balloons. While air-filled balloons do not float, they can be used effectively for decoration purposes at events and celebrations. Helium balloons are popular for their buoyancy and festive appeal.