 Why did the robot at the stream fall into the mud?
The robots were built to serve Man; to do his work, see to his comforts, make smooth his way. Frowning, worrying about the sounds, Alan momentarily forgot to watch his step until his foot suddenly plunged into an ant hill, throwing him to the jungle floor. Without pausing to think, Alan leaped back, and fell sprawling over a bush just as one of the robots rolled silently up from the right, lowering its blaster barrel to aim directly at his head. The planet's double moon had risen and he could see a robot rolling slowly across the clearing in his general direction, blasting indiscriminately at whatever mind impulses came within its pickup range, birds, insects, anything.
 It fell while trying to chase Alan, who managed to confuse it.
 Alan managed to knock it down with his blaster.  It fell while trying to chase Alan, who managed to confuse it.  The mud blocked its sensors and it did not know how to move properly.  Its signals were disrupted and it malfunctioned.