The Supreme Lord is all pervading,existing everywhere,acting as a
witness for all our activities. In this world an object can be at one
place at a certain point of time but it cant be at two places at the
same time. But the Supreme Lord Sri Krishna is simultaneously present
in the Spiritual world as well as in the material world as a single
person. This aspect of the Lord can be understood only by devotees of
the Lord performing pure Devotional Service to Him.
To understand an object x,we need to study the features of the object.
Similarly to understand the Supreme we should also know the qualities
of the Lord.

In this way the devotees are never lose Krishna and Krishna never
loses His devotees. Krishna sees all our activities. He is within our
soul as well as outside the soul. There's no place where He's not
present.
When mother Yashoda,heard the complaints from Krsna's friends,she
decided to punish Him for His good.And so she asked Krsna if he had
eaten sand. Krsna lied and mother Yashoda, asked Him to open His
mouth. Krsna could not refuse opening His mouth because she would
scold.
When He opened the mouth,the Aishwarya shakti automatically worked to
show the whole Universe within His mouth. Even though Krsna can be
very affectionate with His devotees,He can also show His powers at
times and He doesnt have to even think about arranging all these.His
energies themselves are potent to carry out the pastimes.

So in this way,the Lord showed all that is outside within His mouth.
But mother Yashoda could also see all the planets,sun,mountains and
finally she herself seeing into the mouth of Krsna. How does The whole
Universe can be inside Krsna and How mother Yashoda could bear all
that when Krsna had all the universes within His mouth.This is
difficult to understand for the conditioned souls.But this is the
biggest difference between the soul and the Supreme Lord&this is why
the Lord is worshipable&the goal.

Arjuna was also shown the universal form but He lost the friendship
mood for a while after seeing the universal form&started offering His
respects. But mother Yashoda did not get affected to think that Krsna is Supreme Lord,because it will not allow her to enjoy the loving relation toKrsna. She thought that some demons are troubling or she is dreaming Universe to be in Krsna's mouth.

Notes from HH.Bhanu Swami Lecture SB&BG, Sunday Feast Chennai, 23rd, October 2011
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Yours
Dinesh
Blog:http://dinesh-krsna.blogspot.com