Hinduism, What UDDALAKA is trying to teach his son SVETAKETU here? Read it fully?

UDDALAKA to his son SVETAKETU ?My son, place this salt in this water and in the morning come to me, once again.? Svetaketu did so. In the morning his father said to him, ?That salt that you placed in the water last night, please bring it to me.? Svetaketu looked for it but could not find it, for it was totally dissolved. Then his father said to him, ?Please take a sip from this end. How does it taste?? ?Salty,? Svetaketu replied. ?Please take a sip from the middle. How does it taste?? ?Salty?, he replied again. ?Take a sip from that end. How does it taste?? ?Salty as well? he said.

What we can learn from it? Please explain

He is trying to demonstrate that although you cannot see something, or touch it, it does not mean that it isn't there.

Look at the parallels. We cannot see, or touch God (of any description). Svetaketu could see and touch the salt, so he knows that he placed it in the water. When he went back to the water, although he could no longer see or touch the salt, by tasting it in the water, he knew it was there. God, then, cannot be seen or touched, however can be experienced from any part of the world (all of the water was salty).

It is used by Uddalaka to prove to Svetaketu to prove the existence of God.

How does one take a sip from the middle portion and the end portion???
They did not invent the straw then.

This is teaching us that if you add salt to water it will dissolve and make all of the water salty. The deeper meaning of this is that religion works best when stating the obvious because people who believe in god are not that bright and they can only grasp the most basic concepts.

TAT TVAM ASI - You are that, is the second goal in OM TAT SAT .

The father concludes by stating, tat TVA AIs staked... you are that...

The father wants to explain is son that he is not the temporary body that will perish, but he is the eternal spirit soul dwelling within the body that is not seen by material eyes just as salt is not seen.

But the saltiness is tasted and can conclude that salt does exist and did not dissolve itself in the water or the salt did not disappear. If the water evaporates, the salt is left behind to prove that the salt exists.

One passes from various bodies, baby, childhood, boyhood, youth, middle age, old age without seeing the spirit soul within the body just as stakeout did not see the salt in the water. At the time of death the unseen soul goes on to another body. His father is trying to explain his son that he is that spirit soul unseen by material eye.

Moreover, it doesn't stop with that, you are that is expanded to mean, you are spiritual in nature same as the supreme spirit the god. You and god are both same spirit in quality but different in quantity.

A drop of water is salty and the whole ocean is also salty, but the quantity is different, similarly the same thing with Iva (an individual soul like stakeout or anyone) and god.

OM TAT SAT

The three words om tat sat are uttered in conjunction with the holy name of the Supreme Lord, e.g., om tad vising. Whenever a Vedic hymn or the holy name of the Supreme Lord is uttered, om is added. This is the indication of Vedic literature.

These three words are taken from Vedic hymns.
1) Om ity etad brahmano nedistham nama (Rg Veda) indicates the first goal
2) tat tvam asi (Chandogya Upanisad 6.8.7) indicates the second goal
3) sad eva saumya (Chandogya Upanisad 6.2.1) indicates the third goal.

Combined they become om tat sat. Formerly when Brahma, the first created living entity, performed sacrifices, he indicated by these three words the Supreme Personality of Godhead.
http://vedabase.net/bg/17/23/en2