What holds a soap bubble round?

A bubble can leave a square wand and still become round. The wand makes an opening through which a film can swell, but it does not give a free bubble a permanent shape. Once the bubble floats away, the thin film and the air it encloses settle into a shape of their own.

The bubble is mostly gas. Its shell is a very thin layer of soapy water, with air against both its inside and outside surfaces. Water molecules attract one another. At a liquid surface, those attractions give the film a tendency to pull into a smaller area. This effect is called surface tension. Stretching the surface takes energy.

Air trapped inside prevents the film from simply shrinking to nothing. Gas molecules keep moving and striking the film, producing pressure. For a resting round bubble, the inside pressure is slightly greater than the outside pressure. This difference balances the film's tendency to contract. A sphere encloses a given volume using the least possible surface area, so an isolated, undisturbed bubble tends toward that shape.

Soap helps the watery film last long enough to stretch and close around the gas. It actually lowers water's surface tension; it does not make the surface pull harder. The soap molecules gather at the film's surfaces and help stabilize it. A bubble still has surface tension, even though adding soap has reduced it.

A smaller spherical bubble needs a greater difference between inside and outside pressure if the film tension is the same. Small does not automatically mean weakly squeezed. Moving air can stretch a bubble, and touching other bubbles can give parts of it flat faces. Its shape is a visible record of forces acting on an almost invisible sheet of water.