Helium balloons float for the same reason a beach ball floats in water. The balloon and the air inside it weigh less than the equivalent amount of water around it. As a result, the beach ball displaces the water surrounding it, pushing it away as it rises to the surface – a concept known as buoyancy. The same thing happens with a helium balloon. Helium is a special gas that weighs less than the surrounding air: Just like the beach ball in the water, the helium balloon pushes away the surrounding gas and rises above it.
Picture Credit : Google