I'll tell you story.
The confrontation of Arjuna and Ashwatthama in Mahabharata, where Arjuna retracts his weapon as ordered, but Ashwatthama, unable to do so, instead sends it to attack Arjuna's unborn grandson, Parikshit, who is subsequently saved by Krishna. Ashwatthama did not have his bow and arrow near him when he was confronted by Arjuna. So he took a piece of straw and after silently invoking the phrase he threw the straw at Arjuna, which carried the power of the Brahmastra. In response, Arjuna also invoked his Bramhastra to counter the other. But the collision of two Bramhastras would have destroyed the universe; so, Vyasdev came between the two Bramhastras and prevented the collision. Arjuna called back his Bramhastra, but Aswathama did not know this art. So, he wished to change the target and charged Parikshit, the unborn grand-child of Arjuna.