What makes fermenting beer churn?

[NorCalAngler]

Interesting coincidence happened to me this morning. I woke up to yeast filling my airlock. Apparently US-05 loves this SN ESB clone I made. Anyway, the airlock was pretty clogged and I noticed there was much less churning happening in my beer. When I popped the bung to clean it out there was a LOT of pressure in there. I was extremely lucky it didn't blow out. I put a piece of sanitized foil on the carboy, cleaned the airlock and put it back on. I continued getting ready for work and by the time I came back downstairs about 1/2 hour later the churning was going full force again. This is additional evidence it's CO2 propelling the beer debris around. Under pressure and without room for CO2 to escape the activity slowed tremendously.

 

[TANSTAAFB]

The yeast farts co2 so it propels them around. Like blowing up a balloon and then just letting go of the end before tying it. That's what I tell everybody that asks me..

AAANNNDDD I think we have our "break it down Barney style" winner