Soda is fizzy because it contains carbon dioxide, a gas that is put into the bottle at the factory by a special machine. A small amount of carbon dioxide would be able to dissolve in the soda, but since the idea is for it to be bubbly when you drink it, the special machine forces in enough extra carbon dioxide that it creates bubbles! Since this extra carbon dioxide can’t dissolve, it constantly wants to “escape” from the bottle. When the bottle or can sits still on a shelf or a table, a small portion of the carbon dioxide makes its way up to the top of the container. – This is what escapes with a little “hiss” when you first open your soda.
However, if a bottle or can of soda is shaken shortly before it’s opened, something different happens to the contents inside. The carbon dioxide that had been sitting patiently at the top of the container gets mixed around with the liquid soda below it. Because the gas is not sitting by itself at the top of the bottle anymore, when the bottle is opened, it doesn’t escape by itself either! Instead, the bubbles come rushing out from all different parts of the inside of the container and bring with them the liquid soda that they’re now mixed up with!