Prajapati had three kinds of offspring - gods, men and demons (asuras). They lived with Prajapati, practising the vows of Brahmacharins. After finishing their term, the gods said to him - "Please instruct us, Sir." To them he uttered the syllable "Da" and asked - "Have you understood?" They replied - "We have. You said to us, 'Control yourselves (Damyata).'" He said - "Yes, you have understood."
Then the men said to him - "Please instruct us, Sir" To them he uttered the same syllable "Da" and asked - "Have you understood?" They replied - "We have. You said to us, 'Give (Datta).'" He said - "Yes, you have understood".
Then the demons said to him - "Please instruct us, Sir." To them he uttered the same syllable "Da" and asked - "Have you understood?" They replied - "We have. You said to us 'Be compassionate (Dayadhvam).' He said - "Yes, you have understood." That very thing is repeated even today by the heavenly voice, in the form of thunder, as "Da," "Da," "Da," which means - "Control yourselves," "Give," and "Have compassion." Therefore, one should learn these three - self-control, giving and mercy.