Why is Diet Coke fizzy?

The fizz in Diet Coke comes from dissolved carbon dioxide gas (CO2). At room temperature, the pressure inside a sealed bottle of Diet Coke keeps the CO2 dissolved in the liquid. When the bottle is opened or poured, the pressure is released and the CO2 quickly comes out of solution, forming bubbles and releasing the distinctive "sizzle" and bubbles as they rise to the surface. This results in Diet Coke being fizzy in a glass or in your mouth.