हंस: श्वेतो बक: श्वेतो को भेदो बकहंसयो: | नीरक्षीरविवेके तु हंस: हंसो बको बक: || It is said that, if one dilutes milk with water, and gives to swan, swan is able to extract milk and drink it. (dudh ka dudh pani ka pani) Swan is white, crane is (also) white. (then) what is the difference between crane and swan? when it comes to extracting milk from a mixture, swan is swan and crane is crane. i.e. crane does not have this ability. This subhashit is trying to explain that the external appearance does not make a person great but his qualities. It also explains that wise people precisely know what is useful and what is not.