Eli5: why does beer foam up when poured too quickly?

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Answer 1

Beer does have CO2 dissolved into it. The CO2 is a natural product of yeast fermentation. The CO2 can also be forced into flat beer. Head retention is determined mostly by the protein content of beer. This varies based on the recipe. When beer is poured quickly the sudden agitation and the extra space allow the CO2 to escape the liquid that they are dissolved in.

I’ve done a good bit of home brewing. Controlling the carbonation levels to get just the right amount of bubbles is tricky. I’ve had a few foamy disasters that tasted good.