Hot-air balloons are the oldest type of flight technology (pre-dating the airplane by more than 120 years) for a reason: the principle behind “lighter-than-air” flight is simple. Heated air expands, becoming less dense – or lighter – than cool air, which is why hot air rises and cold air falls. Hot-air balloons carry a heat source (typically a flame) that warms the air inside the balloon is lighter than the air outside of it, and the balloon takes flight.
To keep floating, the pilot continues to fire the burners to keep the air inside the balloon hot. To land, the pilot lets the air inside the balloon cool. When this happens, the molecules slow down and take up less space. The air inside the balloon is no longer less dense than the air outside. The balloon sinks towards the ground. So hot air balloons float because hot air is less dense than cold air.
Picture Credit : Google