[Instant Egghead]

[Clink]

That is a terrible thing to do to a beer.

All that delicious, malty goodness gone to waste.

But, why should a tap on top trigger a foam explosion,

while a clink on the side signals the start of a drink with friends?

It has to do with the shape of a beer bottle.

When you "bonk" a beer bottle on top,

the hit creates pressure waves that travel down through the liquid.

These waves bounce off the bottom of the bottle, head back up to the top,

and then bounce back down again.

The changing pressure in the beer rapidly expands and contracts any small bubbles.

At a certain point, these bubbles burst under the pressure,

creating thousands of tiny new bubbles.

Here's where the chain reaction starts.

Each one of these tiny bubbles has a lot of surface area for its volume,

so it's easy for the carbon dioxide dissolved in the beer to get into the bubble.

The bubbles get bigger, getting lighter as they go,

which makes them rise towards the top.

As they rise, they pass through new parts of the liquid,

with lots of spare carbon dioxide.

A feedback loop starts:

the bigger the bubbles get, the faster they rise,

the more CO2 they absorb, which makes them get even bigger.

The process is not unlike the mushroom cloud of the nuclear bomb-

the violent result of an uncontrolled chain reaction,

and, depending on one's mood, only slightly less tragic.

For Scientific American's Instant Egghead, I'm Michael Moyer. [Music]

[English subtitles by moomoo & NagySandor.EU]