Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 19

Sanskrit:

स मामचिन्तयद् देव: प्रश्न‍पारतितीर्षया ।तस्याहं हंसरूपेण सकाशमगमं तदा ॥ १९ ॥

ITRANS:

sa mām acintayad devaḥpraśna-pāra-titīrṣayātasyāhaṁ haṁsa-rūpeṇasakāśam agamaṁ tadā

Translation:

Lord Brahmā 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 Haṁsa, I became visible to Lord Brahmā.

Purport:

Haṁsa 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 Kṛṣṇa appeared as Haṁsa, or the swan, in order to separate the pure consciousness of Lord Brahmā from the modes of material nature.