Verse 6.7.1

षोडशकलः सोम्य पुरुषः पञ्चदशाहानि माशीः काममपः पिबापोमयः प्राणो नपिबतो विच्छेत्स्यत इति ॥ ६.७.१ ॥

ṣoḍaśakalaḥ somya puruṣaḥ pañcadaśāhāni māśīḥ kāmamapaḥ pibāpomayaḥ prāṇo napibato vicchetsyata iti || 6.7.1 ||

1. O Somya, a person has sixteen parts, [and all your sixteen parts are intact]. Do not eat anything for fifteen days, but drink as much water as you like. Life is dependent on water. If you do not drink water, you will lose your life.

Word-for-word explanation:

Somya, O Somya; puruṣaḥ, a person; ṣoḍaśakalaḥ, has sixteen parts; pañcadaśa ahāni, for fifteen days; mā aśīḥ, do not eat; apaḥ piba, drink water; kāmam, as much as you like; prāṇaḥ āpomayaḥ, life is dependent on water; na pibataḥ, if you do not drink water; vicchetsyate iti, it will leave.

Commentary:

Now the son says, ‘I still can’t understand.’ So the father tries something else. A normal human being in good health is said, in Vedānta, to be made up of sixteen parts. If a person neither eats nor drinks water for fifteen days, he will die. But if he does not eat yet continues to drink water during that time he will remain alive. When Gandhiji used to fast, he would drink plenty of water and would put some lemon juice in it. That would keep him alive. But what happens to a person when he drinks water but does not eat? His mind fails. He cannot remember anything.

So, to convince his son, the father asks him to fast for fifteen days but to drink water.