To hide his surprise Sherman bent his head to examine the object the ape-man had handed him. It was about the size of a baseball with little holes in it. He inserted a finger in one of the holes, and a stream of oil squirted out and struck him in the eye. His neighbor gave a cry of annoyance at his clumsiness and reached through the bars to have the ball returned. As he received it there came sudden flickerings of lights along the hall from somewhere high up, like the trails of blue and green rockets. The mechanical ape-man dropped the oil-ball and dashed to the front of his cell. Sherman saw a vehicle proceeding down the line of cells; a kind of truck that rode on the track of the corridor and was so wide it just missed the gratings. It had a long series of doors in its sides, and as it came opposite an occupied cell, stopped. Something invisible happened; the bars of the cell opened inward and the inmate emerged to step into a compartment which at once closed behind him. When it stopped at the ape-man's cage Sherman watched the procedure closely. A little arm appeared from beneath the door of the compartment and did something to one of the lower bars of the cell. But the truck passed Sherman by, moving silently along to other cells beyond him. He turned to examine the room more closely, and as he did so, saw that a second truck was following the first. This one, with an exactly reversed procedure, was returning robots to their cells. This second truck dropped an inmate in the cell at his right (another ape-man) and trundled along down the line, but as it reached the end of the corridor, turned back and running along till it came to his cell, stopped, flung out the metal arm, and opened the bars in invitation. Sherman had no thought of disobeying; as long as he was in this queerest of all possible worlds, he thought, one might as well keep to the rules. But he was curious about the joint of the cage and how it unlocked and he paused a moment to examine it. The machine before him buzzed impatiently. He lingered. There came a sudden clang of metal from inside the car, a vivid beam of blue light called his attention, and looking up, he saw the word "EXIT" printed in letters of fire at the top of the compartment. With a smile he stepped in. A soft light was turned on and he found himself in a tiny cubbyhole with just room for the single seat it provided and on which he seated himself. There was no window. The machine carried him along smoothly for perhaps five minutes, stopped and the door opened before him. He issued into another blue-domed hall. A small one this time, containing a rubber seat like that in his cell, but with an extended arm on which rested a complex apparatus of some kind. The seat faced a white screen like those in movie theaters.