Bubbles can also be a thin, ball-shaped film of liquid that has gas trapped inside.
Harder - A bubble is a spherical mass of gas surrounded by a liquid or solid. Soap bubbles are shaped by an equilibrium between their outward air pressure and the inward surface tension of the soap film. The detergent molecules tend to line up with one end pointing inward and the other outward, making the liquid surface more stable. The thickness of the soap film layer is due to these detergent molecules and is uniform for all size bubbles. Colors are seen because of the light diffraction through the soap film.