system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does shaking up a soda increase the pressure inside the bottle?

Obviously it’s science, but I can’t see how it happens because the carbonation is already in the container with the soda. Even if it separates, the volume should be the same... Right?
gpt
It's because the carbonation actually has two different states, a gas and an acid.

When you take carbon dioxide gas and put it under pressure next to a liquid like water or soda or cider or beer, that CO2 actually reacts with the water to form a fairly unstable acid called "carbonic acid". 

It's unstable enough that as soon as the pressure goes away, the carbonic acid molecules start breaking back down into water and carbon dioxide molecules. Open a carbonated beverage container and over a little while the breakdown transforms all of the carbonic acid back to carbon dioxide, the carbon dioxide bubbles off, and the container of liquid goes flat. 

And a shock makes the instability a lot worse. If you increase the instability by pouring a just-opened beverage from on high, the shock causes the carbonic acid to rapidly break down, which is why pouring soda too fast causes a huge foam-up. Ditto when you tap a glass of beer with another glass of beer.

And if you shake the bottle or can of soda without opening it, you're breaking a whole bunch of carbonic acid into carbon dioxide - a gas - and it really increases the pressure in the can because all that CO2 can't leave. But wait a while, and the pressure will force the CO2 to form back into carbonic acid... and the pressure will reduce so it won't blow up when you open it.