Why do some people knock on the lid of a beverage can?
News News blog
Some people knock on wood, but other people knock on the lid of a beverage can. Is that a superstition?
Beverage cans are practical and easy to transport, but if the can has been shaken, it foams over when opened.
When a can is shaken, small bubbles form on the inside rim of the can. If the can is then opened, the high pressure drops - the bubbles get bigger, push upwards and the can foams over.
There is a simple trick to avoid this: many people tap the lid of the can with their finger before opening it. Even better: flick the side of the can vigorously with your finger - from top to bottom, all the way around the can.
This causes the bubbles to detach from the inside edge of the can and rise to the top. When the can is then opened, you only hear a slight hissing sound, but the drink remains in the can.
