खलः सत्क्रियमाणोपि ददाति कलहं सतां
दुग्ध धौतोपि किं याति वायसः कलहंसताम
Khalah satkriyamaanopi dadaati kalahh sataam
dugdh dhotopi kim yaati vayasahh kalhanstaam
i.e. A mean person inspite of being helped and shown kindness to him, in return creates quarrel and conflict, just like a crow which can not become white inspite of being repeatedly washed with milk.