Why does soda fizz?
The Real Answer
Soda fizzes because it contains carbon dioxide gas dissolved in the liquid under pressure. While the bottle or can is sealed, the higher pressure helps keep more of that carbon dioxide dissolved.
When you open the container, the pressure above the liquid drops. Carbon dioxide can then leave the solution more easily. Gas molecules collect at tiny imperfections and other nucleation sites, forming bubbles that grow, rise, and release carbon dioxide into the air.
That same process is why an opened soda gradually goes flat: over time, more of its dissolved carbon dioxide escapes until there is much less gas left to make bubbles.
Opening a soda turns one pull-tab into a two-stage decision. You crack it just enough to hear the hiss, stop for one second, then finish opening the exact same can and act surprised when the foam keeps coming.
Why People Ask This
People ask because a sealed soda looks completely calm, then immediately starts hissing and bubbling when opened. The sudden change makes it seem as though the bubbles were somehow trapped inside in visible form, when the more useful explanation is that dissolved carbon dioxide begins escaping as the pressure changes.