Dronacharya, taught archery to the Kauravas and Pandavas. Dronacharya had a son named Ashwatthama. Ashwatthama also learned archery from his father Dronacharya and his maternal uncle Kripacharya, and became a great archer himself. He learned many secret ways to use the bow and arrow, and soon became an expert. Ashwatthama knew that Shrikrishna had blessed the Pandavas. So he thought, “This is the time for me to go to Shrikrishna and get something from him.”
He went to Shrikrishna and said, “I can give You my most powerful weapon, the Brahmashira. It can kill anyone against whom it is used. Will You give me Your Divine disc (Sudarshan Chakra) in return ? Will you trade it with me ?”
Shrikrishna said, “Wonderful! I am ready to exchange it. Please take it.” So saying, He put forth the Sudarshan Chakra for Ashwatthama to take. Immediately, Ashwatthama tried to take the Chakra, but it was impossible for him to lift it, let alone move it.
Shrikrishna smiled, “Young man, you cannot even lift my weapon. How are you going to use it?” Ashwatthama was embarrassed, and left red faced.