Why does soda fizz?

Soda fizzes because carbon dioxide gas is squeezed into the drink while the container is sealed. When you open it, the pressure drops, and the gas can leave the liquid. It gathers into bubbles, rises, and escapes into the air with that familiar hiss.

Those bubbles often begin on tiny rough places, such as a scratch in a glass or the surface of ice. Each spot gives the gas a starting point. Once the bottle or can stays open, the carbon dioxide slowly leaves, and the drink becomes flat.