The air is heated by the burner and the expanding air becomes less dense, causing the balloon to rise through the denser, cooler surrounding air.
(The air; is heated; by the burner) (cooler; surrounding; air) (the expanding air; becomes; less dense) (the expanding air; causing; the balloon to rise through the denser) Context(the expanding air causing,List([36, 81))):(the balloon; to rise; )
the expanding air causing,List([36, 81))
The air cooler the expanding air the expanding air the balloon
is heated surrounding becomes causing to rise
by the burner
air
less dense
the balloon to rise through the denser