The gas that lifts a balloon into the air is lighter than air. The most common gas used is helium (He), but hydrogen (H2) was also used historically.

Here's why this works:

* Buoyancy: A balloon floats because it displaces a volume of air that weighs more than the balloon itself. This is the same principle that makes ships float on water.

* Density: Gases like helium and hydrogen have a much lower density than air. This means that a certain volume of these gases weighs less than the same volume of air.

* Lifting Power: The difference in density between the lifting gas and air creates a force called "lifting power." This force is what pushes the balloon upward.

Why Helium is preferred over Hydrogen:

* Safety: Hydrogen is extremely flammable, making it a safety hazard.

* Availability: Helium is readily available and relatively inexpensive.

Let me know if you'd like to learn more about how balloons work!