Krausen

When the beer first starts to ferment, little circles of foam form up, expanding to eventually cover the surface. This first shot is about five hours after pitching.

Then we have it at about 12 hours. We have a dense, rocky head of foam. Which gets taller another 24 hours later (the third pic), which is what's known as high krausen.



The is the reason for using acid carboys for primary fermenters: they have enough headroom to allow this to happen without the foam coming up through the airlock and making a big mess.

Or, likely as not, throwing the airlock completely. The proteins and whatnot that are floating on the foam, they clog the airlock whent he foam gets there, and a little bit more C02 builds up and the airlock flies like a champagne cork.