Science Background Balloons and blimps fly because of the physical differences between different gases and how gases change with temperature . When the gas inside a balloon has a lower density than the surrounding gases, the balloon gains buoyancy . Some gases, like helium , have a lower molecular weight and rise up through the heavier air.
Gases are the most energetic of the three phases of matter.
Gas molecules have little attraction to each other and are spread far apart. Increasing the temperature of a gas, increases the molecular energy, pushing the molecules farther apart. This decreases the density of the gas. Hot-air balloons hold hotter and therefore lighter air inside their envelopes and fly because of the buoyant force pushing the lighter hot air up.