kasmad dadhara go-rupam
yam dudoha prthus tatra
ko vatso dohanam ca kim
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Vidura inquired from the great sage Maitreya: My dear brahmana, since mother earth can appear in different shapes, why did she take the shape of a cow? And when King Prthu milked her, who became the calf, and what was the milking pot?