Here's why:

* Density: Density is a measure of how much mass is packed into a given volume. Denser things are heavier for their size.

* Air: Air is a mixture of gases, primarily nitrogen and oxygen.

* Helium: Helium is a much lighter element than nitrogen and oxygen.

* Buoyancy: When an object is placed in a fluid (like air), it experiences an upward force called buoyancy. This force is equal to the weight of the fluid displaced by the object.

* The Rise: Because helium is less dense than air, a helium-filled balloon displaces more air than its own weight. This creates a buoyant force that is greater than the balloon's weight, causing it to rise.

In simpler terms: Imagine a helium balloon as a tiny boat. The helium inside is like the boat's hull, displacing the heavier air around it. Because the displaced air is heavier, the balloon is pushed upwards.