Compliments TLC Cooking

go here if you wanan read the full thing... http://recipes.howstuffworks.com/question602.htm/ but basically:

People tap on soda cans to avoid a soda explosion from a can that has been shaken up. Tapping may, in fact, decrease the likelihood of such an explosion, but a positive outcome is not guaranteed. When you tap on the can, you knock bubbles off the bottom and sides of the can, at which point they rise to the top (because gas is lighter than liquid) and there is only a small amount of liquid blocking their escape when you open the can. Unfortunately, there's no way to tell how many of the bubbles you've displaced in your tapping effort

hope that helps out a lil

^^ dude beat me to it but w/e