If you tap the top of a soda can, will it really keep it from foaming when you open it?
Those who think so follow this logic: When you shake a soda can, little molecules of carbon dioxide bump into each other and form bubbles. These bubbles hang around on the sides and bottom of the can until you pop the top, at which time they shoot out, eager to escape to the lower air pressure outside the can. Some liquid clings to them on their way out—and winds up all over your shirt. Tapping the can prior to opening it, the theory goes, dislodges bubbles from the bottom and sides. The bubbles join the big pocket of gas at the top, which escapes harmlessly because it’s above the liquid.
But the theory is wrong. Doesn’t work. “We are not aware of any impact that tapping on the top of a can before you open it has on fizzing over,” said Scott Williamson, a Coca-Cola spokesman in Atlanta. A counterpart at Pepsi concurred, though neither was quite able to explain why the tapping doesn’t work.
It even turns out it could do more harm than good. “If you were tapping rigorously enough to dislodge bubbles from the bottom and the side, you risk creating more bubbles,” said Karl J. Siebert, a biochemistry professor and foam expert in Cornell University’s Food Science & Technology Department.
So tap at your own risk.