What is a Bubble

Because liquid molecules are strongly attracted to each other; the surface of a liquid—the place where it meets air and other gases—will shrink to the smallest possible area.
For example, when air is injected into water; the water molecules cling to each other and minimise their contact with the air.
The smallest possible surface area required to enclose the air happens to be a sphere: a bubble.
The bubble, weighing far less than the water it displaces, rises to the surface. Competitive high divers sometimes practice new moves using a bubble machine that sends a stream of air up from the bottom of the swimming pool; this creates a more forgiving, cushioned landing zone by continuously disrupting the water's surface tension.