As per this story in the Puranas, Ashwatthama, son of Dronacharya, wanted to possess the
Sudarshana Chakra of Sri Krishna and add it to his array of weapons.
Ashwatthama was one of the greatest archers and he had learned the art of
archery from his father, Dronacharya, and uncle, Kripacharya. He had the
capacity to use several rare weapons and arrows. He also had in his possession
the powerful Brahmastra.
But Ashwatthama was never satisfied with what he had. He
always knew that the Sudarshana Chakra of Sri Krishna was more powerful than
all the weapons and arrows he possessed.
The desire to acquire Sudarshana Chakra increased day by day
and finally Ashwatthama decided to ask for it.
He went to Sri Krishna and told him that he is ready to give
to him all the secret weapons and arrows that is in his possession. He told he
is even ready to give away his Brahmastra provided Sri Krishna gave his
Sudarshana Chakra to him.
Sri Krishna smiled and agreed to the suggestion of
Ashwatthama.
Sri Krishna then brought forth the Sudarshana Chakra and
placed it on the ground.
Ashwatthama immediately went to take it up. To his surprise
he could not lift it up. He was embarrassed and ashamed that he could not even
move it a bit.
Ashwatthama realized the power of Sri Krishna
and Sudarshan Chakra and left the place embarrassed.