My explanation for why soda fizzes:

The little bubbles in soda are made of carbon dioxide that is injected into the drink when it is bottled. The little space of air at the top of the bottle or the can has carbon dioxide that wasn’t mixed with the drink, so it makes the ‘pish’ noise when you first open it. Once the soda can or bottle is opened, the carbon dioxide that is in the liquid is able to escape back into the air, causing bubbles to appear in the liquid and work their way to the top.

When the bottle or can is still sealed, there is equal pressure holding all of the gas in the canister.