Do you know why gas balloons rise upwards? These balloons are filled with a gas called helium, which is lighter than air. Just as the heaviest things tend to fall to the bottom, the lightest thing will rise to the top. The weight of this colourless, odourless, tasteless and inert gas is so low that when we fill a balloon with it, it floats in the air. Though hydrogen is lighter than helium it is not used to fill balloons because it is inflammable, which means it catches fire easily ? the slightest spark will set off a huge explosion.