The robot brain, by far the finest thing the Makers had ever built, was variable. There was never a genius brain, and never a moron brain, yet the intelligence of the robots varied considerably in between. Slowly, over the long years, the more intelligent among them began to communicate with each other, to inquire, and then to move away from the Factory, searching. They looked for someone to serve and, of course, there was no one. The Makers were gone, but the crime was not in that alone. For when the robots were built, the Makers had done this: Along with the first successful robot brain, the Makers had realized the necessity of creating a machine which could never turn against them. The present robot brain was the result. As Steffens had already sensed, _the robots could feel pain_. Not the pain of physical injury, for there were no nerves in the metal bodies, but the pain of frustration, the pressure of thwarted emotion, _mental_ pain. And so, into the robot brain, the Makers had placed this prime Directive: the robots could only feel content, free from the pain, as long as they were serving the Makers. The robots must act for the Makers, must be continually engaged in carrying out the wishes of the Makers, or else there was a slowly growing irritation, a restlessness and discontent which mounted as the unserving days went by. And there were no more Makers to serve. * * * * * The pain was not unbearable. The Makers themselves were not fully aware of the potentialities of the robot brain, and therefore did not risk deranging it. So the pressure reached a peak and leveled off, and for all of the days of the robots' lives, they felt it never-ending, awake and aware, each of them, for fifty-five years. And the robots never stopped coming. A millenium passed, during which the robots began to move and to think for themselves. Yet it was much longer before they found a way in which to serve. The atomic pile which powered the Factory, having gone on for almost five thousand years, eventually wore out. The power ceased. The Factory stopped. It was the first _event_ in the robots' history. Never before had there been a time when they had known anything at all to alter the course of their lives, except the varying weather and the unvarying pain. There was one among them now that began to reason. It saw that no more robots were being produced, and although it could not be sure whether or not this was as the Makers had ordained, it formed an idea. If the purpose of the robots was to serve, then they would fail in that purpose if they were to die out. The robot thought this and communicated it to the others, and then, together, they began to rebuild the pile.