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SB 10.3.46

śrī-śuka uvāca
ity uktvāsīd dharis tūṣṇīṁ
 bhagavān ātma-māyayā
pitroḥ sampaśyatoḥ sadyo
 babhūva prākṛtaḥ śiśuḥ
Synonyms: 
śrī-śukaḥ uvāca — Śrī Śukadeva Gosvāmī said; iti uktvā — after instructing in this way; āsīt — remained; hariḥ — the Supreme Personality of Godhead; tūṣṇīm — silent; bhagavān — Lord Viṣṇu, the Supreme Personality of Godhead; ātma-māyayā — by acting in His own spiritual energy; pitroḥ sampaśyatoḥ — while His father and mother were factually seeing Him; sadyaḥ — immediately; babhūva — He became; prākṛtaḥ — like an ordinary human being; śiśuḥ — a child.
Translation: 
Śukadeva Gosvāmī said: After thus instructing His father and mother, the Supreme Personality of Godhead, Kṛṣṇa, remained silent. In their presence, by His internal energy, He then transformed Himself into a small human child. [In other words, He transformed Himself into His original form: kṛṣṇas tu bhagavān svayam.]
Purport: 

As stated in Bhagavad-gītā (4.6), sambhavāmy ātma-māyayā: whatever is done by the Supreme Personality of Godhead is done by His spiritual energy; nothing is forced upon Him by the material energy. This is the difference between the Lord and an ordinary living being. The Vedas say:

parāsya śaktir vividhaiva śrūyate
 svābhāvikī jñāna-bala-kriyā ca

(Śvetāśvatara Upaniṣad 6.8)

It is natural for the Lord to be untinged by material qualities, and because everything is perfectly present in His spiritual energy, as soon as He desires something, it is immediately done. The Lord is not a prākṛta-śiśu, a child of this world, but by His personal energy He appeared like one. Ordinary people may have difficulty accepting the supreme controller, God, as a human being because they forget that He can do everything by spiritual energy (ātma-māyayā). Nonbelievers say, “How can the supreme controller descend as an ordinary being?” This sort of thinking is materialistic. Śrīla Jīva Gosvāmī says that unless we accept the energy of the Supreme Personality of Godhead as inconceivable, beyond the conception of our words and mind, we cannot understand the Supreme Lord. Those who doubt that the Supreme Personality of Godhead can come as a human being and turn Himself into a human child are fools who think that Kṛṣṇa’s body is material, that He is born and that He therefore also dies.

In Śrīmad-Bhāgavatam, Third Canto, Fourth Chapter, verses 28 and 29, there is a description of Kṛṣṇa’s leaving His body. Mahārāja Parīkṣit inquired from Śukadeva Gosvāmī, “When all the members of the Yadu dynasty met their end, Kṛṣṇa also put an end to Himself, and the only member of the family who remained alive was Uddhava. How was this possible?” Śukadeva Gosvāmī answered that Kṛṣṇa, by His own energy, destroyed the entire family and then thought of making His own body disappear. In this connection, Śukadeva Gosvāmī described how the Lord gave up His body. But this was not the destruction of Kṛṣṇa’s body; rather, it was the disappearance of the Supreme Lord by His personal energy.

Actually, the Lord does not give up His body, which is eternal, but as He can change His body from the form of Viṣṇu to that of an ordinary human child, He can change His body to any form He likes. This does not mean that He gives up His body. By spiritual energy, the Lord can appear in a body made of wood or stone. He can change His body into anything because everything is His energy (parāsya śaktir vividhaiva śrūyate). As clearly said in Bhagavad-gītā (7.4), bhinnā prakṛtir aṣṭadhā: the material elements are separated energies of the Supreme Lord. If He transforms Himself into the arcā-mūrti, the worshipable Deity, which we see as stone or wood, He is still Kṛṣṇa. Therefore the śāstra warns, arcye viṣṇau śilā-dhīr guruṣu nara-matiḥ. One who thinks that the worshipable Deity in the temple is made of wood or stone, one who sees a Vaiṣṇava guru as an ordinary human being, or one who materially conceives of a Vaiṣṇava as belonging to a particular caste is nārakī, a resident of hell. The Supreme Personality of Godhead can appear before us in many forms, as He likes, but we must know the true facts: janma karma ca me divyam evaṁ yo vetti tattvataḥ (Bg. 4.9). By following the instructions of sādhu, guru and śāstra — the saintly persons, the spiritual master and the authoritative scriptures — one can understand Kṛṣṇa, and then one makes his life successful by returning home, back to Godhead.