You probably know that soda is fizzy because it has lots of dissolved carbon dioxide in it. Normally, though, carbon dioxide is a gas. And if you were to take all of the carbon dioxide out of a can of soda and just let it be a gas, you’d find that it would take up a lot more space than just the can, if it were at ordinary atmospheric pressure. (So when it's dissolved, it takes up less space than when it's a gas.)
If you shake a can of soda, you end up with tiny bubbles of carbon dioxide gas that stick to the inside surface of the can. Remember, though, that these bubbles are under high pressure, so they’ll expand if the can is opened and the pressure falls to ordinary atmospheric levels. If you open the can, the bubbles expand a lot and they push the soda right out with them, causing a "soda explosion."
But if you tap on the bottom of the can, the bubbles get knocked loose of the side of the can and float to the top. So all of the little bubbles form one big bubble at the top. That way the gas still expands when you open the can, but since there’s no soda in the way, the soda doesn’t get pushed out too. So the answer to your question is yes: tapping on the bottom of the can or bottle should help to prevent a "soda explosion."
-Tamara and Mike
(republished on 07/17/06)