Verse 4.14.2

भगव इति ह प्रतिशुश्राव ब्रह्मविद इव सोम्य ते मुखं भाति को नु त्वानुशशासेति को नु मानुशिष्याद्भो इतीहापेव निह्नुत इमे नूनमीदृशा अन्यादृशा इतीहाग्नीनभ्यूदे किं नु सोम्य किल तेऽवोचन्निति ॥ ४.१४.२ ॥

bhagava iti ha pratiśuśrāva brahmavida iva somya te mukhaṃ bhāti ko nu tvānuśaśāseti ko nu mānuśiṣyādbho itīhāpeva nihnuta ime nūnamīdṛśā anyādṛśā itīhāgnīnabhyūde kiṃ nu somya kila te'vocanniti || 4.14.2 ||

2. [Upakosala] replied, ‘Yes, lord.’ [His teacher said:] ‘O Somya, your face is shining like that of a knower of Brahman. Who has taught you?’ ‘Sir, who will teach me?’ He said this as if he was trying to hide the truth. Then, pointing to the fires, he said: ‘Earlier they looked, different. Now they look like this.’ In this way, he indicated the fires. [The teacher asked,] ‘Somya, what did the fires teach you?’.

Word-for-word explanation:

Bhagavaḥ iti ha pratiśuśrāva, ‘Yes, lord,’ he replied; brahmavidaḥ iva somya te mukham bhāti, your face is shining like that of a knower of Brahman; kaḥ nu tvā anuśaśāsa iti, who has taught you; kaḥ nu mā anuśiṣyāt bho iti, who will teach me, sir; iha iva apanihnute, [he said this] as if he was trying to hide [the truth]; ime, these [pointing to the fires]; nūnam īdṛśāḥ, look like this [as if they are frightened]; anyādṛśāḥ iti, looked different before; iha agnīn abhyūde, in this way he spoke about the fires; kim nu somya kila te avocan iti, Somya, what did they teach you?

Commentary:

Upakosala knew the limitations of the fires, and that is why he did not want to tell his teacher what they had taught him, or that they had taught him anything at all. Further, he noticed that in the presence of his teacher the fires appeared to be frozen with fear. This is why he was vague and evasive in his reply.