Advanced sawtooth
In fact, the serrations of beer bottle caps are not always 21, but have been selected and optimized for a long time.
The earliest beer caps did not have serrations, and most "bottle caps" were corks. However, it is very troublesome to open the cork. In addition, the beer itself is full of gas. If it cannot be opened at once, the gas in the beer will slowly run away, unable to form sufficient foam, resulting in poor taste of beer.
So, people have been searching for suitable bottle caps.
