Bubbles form when:

* Gas dissolves in a liquid and then comes out of solution. This happens when the pressure changes (like opening a bottle of soda) or when the temperature increases (like when water boils).

* Gas is injected into the liquid. This is how bubbles are created in things like carbonated drinks or when scuba divers exhale underwater.

* A chemical reaction produces gas. This happens in things like baking soda and vinegar reactions or when yeast ferments.

Bubbles can be different sizes and shapes depending on the gas, the liquid, and the conditions.