Once a dispute arose between vak ( Speech ) and manah ( Mind ) as to who is superior between them. Mind said that I am superior as thoughts originate in me. Speech said I am the one who makes thoughts aware to the outer world. Without me nobody would know about them.
They both approached Prajapati to settle the dispute. Prajapati, after contemplation said that it is the Mind which is superior as Speech is only a follower of the Mind. Upon hearing this, Speech thought that injustice has been done to her and refused any further cooperation with Prajapati in yajnas.
Therafter, any offering made to Prajapati in yajna is done silently.