Swans are revered in Hinduism, and are compared to saintly persons whose chief characteristic is to be in the world without getting attached to it, just as a swan's feather does not get wet although it is in water. The Sanskrit word for swan is hamsa or hansa, and the "Raja Hansa" or the Royal Swan is the vehicle of Goddess Saraswati, and symbolises the "Sattwa Guna" or purity par excellence. The swan if offered a mixture of milk and water, is said to be able to drink the milk alone. Therefore Goddess Saraswati the goddess of knowledge is seen riding the swan because the swan thus symbolizes "Viveka" i.e. prudence and discrimination between the good and the bad or between the eternal and the transient. This is taken as a great quality, as shown by this Sanskrit verse:
Hamsah shwetah, bakah shwetah, kah bhedah hamsa bakayo? Neeraksheera viveketu, Hamsah hamsah, bakah bakah! The swan is white, the crane is white, so how to differentiate between them? With the milk-water test, the swan is proven swan, the crane is proven crane!