sa mam acintayad devah
sakasam agamam tada
Lord Brahma desired to attain the answer to the question that was puzzling him, and thus he fixed his mind on Me, the Supreme Lord. At that time, in My form of Hamsa, I became visible to Lord Brahma.
Hamsa means “swan,” and the specific ability of the swan is to separate a mixture of milk and water, extracting the rich, milky portion. Similarly, Lord Krsna appeared as Hamsa, or the swan, in order to separate the pure consciousness of Lord Brahma from the modes of material nature.