In the Chandogya Upanishad, Shwetaketu could not understand how food became the mind or water prana. Uddalaka decided to explain the matter with a practical example. He asked Swetaketu to avoid taking food for fifteen days and live only on water.
Shwetaketu survived this test as the water he was taking
ensured that prana did not leave the body. But he could not recall the Vedic
mantras that he had earlier committed to memory.
Uddalaka explained: ‘Just as a single ember, the size of a
firefly, remains as the residue of a big fire, and by that nothing bigger is
burnt, even so, my dear, of your sixteen parts only one remains; with that you
are unable to recall the Vedas. Eat, and then you will understand me.’
Shwetaketu started taking food again, and his memory was
revived. He understood how the mind was made of food and the vital force of
water.
Next Uddalaka pointed out that pure Being underlies all
phenomenal objects derived from food, water, and fire. The body is born of food,
and food of water; and water has fire (energy) as its source. Fire, or tejas,
originates from Being, from Brahman. Thus ‘when a person departs hence, speech
is withdrawn into the mind, mind into vital force, vital force into fire, and
fire into the supreme Deity’ (Chhandogya Upanishad 6.8.6). The body of the
dying person stops speaking, fails to respond to commands, becomes bereft of
life, and turns cold in succession. The individual is then merged in Being,
although this merger is only temporary, as in deep sleep. The phenomenal name
and form are transitory and are hence termed unreal, mithya. Sat or Being is
ever-existent: ‘All creatures are rooted in Being, they dwell in Being, and
(finally) merge in Being.
Source – Excerpts from article titled ‘Tat-tvam-asi Shwetaketu’ by Swami Alokananda published in the September 2008 edition of Prabuddha Bharata Magazine.