title_body: Why do stouts foam so much when you pour them over ice cream?
upvoted_answer: For the same reason that root beer floats behave in a similar fashion.  Even though there is less carbonation than in root beer (and I presume similarly less foam, though still a significant quantity) the irregular surface of the ice cream offers a massively larger number of nucleation sites than the smooth glass of your favorite beer vessel.   These nucleation sites cause the CO2 to come out of solution, and voila, much foam does appear.  I believe that the ice cream has properties that enhance the foaming effect as well.