Answer:
The movement of air in a hot air balloon makes it move up into the sky.
Step-by-step explanation:
Convection takes place in fluids. Hot fluids are less dense than cold fluids and the difference in the densities determine the direction of flow of fluids. Less dense fluid moves up and the denser fluid take its place.
Thus hot air rises up and cold air fills its place. The hot air balloon contains burner which pumps hot air into the balloon.
Thus the air inside the balloon will be hotter than the air outside the balloon. Thus the hot air rises up carrying the balloon forward.