Verse 6.5.4

अन्नमयंहि सोम्य मनः आपोमयः प्राणस्तेजोमयी वागिति भूय एव मा भगवान्विज्ञापयत्विति तथा सोम्येति होवाच ॥ ६.५.४ ॥
॥ इति पञ्चमः खण्डः ॥

annamayaṃhi somya manaḥ āpomayaḥ prāṇastejomayī vāgiti bhūya eva mā bhagavānvijñāpayatviti tathā somyeti hovāca || 6.5.4 ||
|| iti pañcamaḥ khaṇḍaḥ ||

4. ‘O Somya, the mind is nourished by food, prāṇa by water, and speech by fire.’ [Śvetaketu then said,] ‘Sir, will you please explain this to me again?’ ‘Yes, Somya, I will explain again,’ replied his father.

Word-for-word explanation:

Somya, O Somya; manaḥ hi annamayam, the mind is nourished by food; prāṇaḥ āpomayaḥ, the vital force is nourished by water; vāk tejomayī iti, speech is nourished by fire; bhagavān, sir; bhūyaḥ eva, again; mā, to me; vijñāpayatu iti, will you please explain; tathā, so be it; somya iti, O Somya; ha uvāca, he [the father] said. Iti pañcamaḥ khaṇḍaḥ, here ends the fifth section.

Commentary:

In the Vedānta philosophy there are said to be five elements. The first two—ākāśa (space, or ether) and vāyu (air)—are not visible to us. The other three—pṛthivī (earth—here referred to as annam, food), āpa (water), and agni, or tejas (fire, or energy)—are visible, and this is why they have been discussed here. These three elements, however, are never to be found in their pure form. When we perceive them, they are always in a combined state. For instance, the water that we see always has some earth and other elements mixed with it. In reality, though, it is pure Spirit which we see as earth, water, or fire.

The father has been telling his son that the mind is the finest product of food, that prāṇa, the vital force, is the finest product of water, and that speech is the finest product of fire. Naturally the son is confused. It is not very clear. So he asks his father to explain it further.