As a child, my source first heard this superstition because he was rather clumsy and would always drop his soda cans. He first instance he can remember where he practiced this superstition was with his teammates from his little league baseball team. All of his teammates would tap on the tops of their soda cans under any condition, just to be sure to limit the fizz. My source suggested that this could have been a popular trend shared by his teammates or the result of several players being chewed out by their parents for spilling soda on their uniforms.
My source doesn’t quite understand how the superstition works, but is assured that it’s effective. After tapping the top of his soda cans he has rarely ever had a spill. If he had to guess, he supposes that when the can is tapped, carbonation bubbles stuck to the side of the can are nudged to the top of the can, and when the can is opened, the gas is released without dragging any liquid out with it. My source has also heard of a ‘three-tap method,’ where the can needs to be tapped only three times, but he is not sure where he heard this.
Scientifically, when a can is dropped, the carbon dioxide that carbonates the soda is forced out of the liquid and into a gas form which builds pressure in the can. If the can is opened soon after, the pressure will be released and the gas will rush out, dragging the soda with it. If somebody taps on a soda can, that’s an added disturbance, which would likely cause more of the carbon dioxide to be freed from the liquid, so the superstition should not work. What does work is waiting for the pressure to be reduced. After a drop, given time, the carbon dioxide will assimilate back into the liquid, reducing the pressure and fizz when the can is opened. Also, because of the built up pressure, the rate at which one opens a can is significant because it regulates the speed any carbon dioxide is released. Finally, there is a theory that if you tap a can with a metal rod, it will create a vibration in the aluminum can that will cause all of the gas to move to the top of the can, reducing potential fizz, but this method isn’t proven.
From my own personal experience, I have experienced good results after I’ve tapped the top of my dropped soda can, but I cannot attribute it to the superstition. I believe that by tapping the top of the soda can, you’re spending more time not opening the can. As long as you’re tapping, the soda can isn’t opened, and this gives the carbon dioxide more time to be pushed back into the liquid.
Annotation: A variant of this superstition is featured in episode 513 (My Five Stages) of Bill Lawrence’s NBC sitcom Scrubs as the “John Dorian three-tap method. Three taps and the foam goes bye-bye.” The character, JD, then opens the can, and after a pause, all hell breaks loose and foam flies everywhere.