The bubbles in soda come from a gas called carbon dioxide that is dissolved into the soda. Out in the open, carbon dioxide prefers to be a gas, but inside a soda bottle where the pressure is high, it's forced to be a liquid. (Dissolved carbon dioxide turns into carbonic acid, which is why soda is so bad for your teeth. Drinking diet soda doesn't help because it also contains this acid.) When you take the top off of a bottle of soda, the pressure inside the bottle decreases and goes to the same pressure as the atmosphere. When that happens the carbon dioxide inside is no longer forced to be a liquid and turns back into a gas, causing the bubbles that we're so familiar with. If a soda sits out for a long time eventually all of the carbon dioxide will turn to gas and leave the soda flat. If you look closely, you'll notice that most of the bubbles in a soda are at the top, the surface that's open to the air, that's why it takes so long for a soda to go flat. Only the bubbles at the top get to escape.

Shaking a bottle of soda mixes up all the bubbles from the surface and they get distributed all throughout the soda and stick to the sides of the bottle as well. Then, when you open the bottle all the bubbles float to the surface at once and there's a bit of a traffic jam. There are so many bubbles all going to the surface at the same time that soda gets trapped between them and carried out the top with the gas.

Most carbonated drinks have dissolved CO₂ (carbon dioxide).When the soda is made, a lot of CO₂ is dissolved into the liquid. After the bottle is sealed, the CO₂ is trapped in the liquid, but would rather leave the solution. Therefore an equilibrium is established between the amount of CO2 in the liquid and the pressure of CO₂ gas in the top of the bottle. When you open the bottle, there is a dramatic decrease in pressure over the liquid, so the CO₂ starts to leave the liquid very rapidly, causing the mass exodus of gas, or "explosion" of bubbles. This also explains why soda goes flat. Given enough time, the CO₂ will all leave the solution, removing all of the dissolved gas which gives the soda its fizziness.

The carbonation is due to the presence of dissolved carbon dioxide gas. The gas exerts pressure when not dissolved, which can cause it to burst from its container if too much is released. If exposed to air, the gas will slowly come out of solution and be released to the atmosphere, causing the drink to go flat.