An initially vigorous pouring action develops the head of foam with a small, easily controlled portion of the beer. Once the foam is of the desired thickness, the rest of the beer can be poured in gently along the side of the glass, avoiding aeration and nucleation of new bubbles. The glass itself should be clean of any residues of oil or soap, which interfere with foaming. (These molecules have water-avoiding ends that pull the similar ends of the bubble-stabilizing proteins out of the bubbles.) By the same token, if a newly poured beer threatens to foam over, it can often be stopped in its tracks by touching the rim with a finger or lip, which carry traces of oil.