When he woke up one morning Robin was vaguely aware of something different. He opened his eyes to the dark interior of his cave-home and lay there on his bed of padding from the cargo chamber. For a while he rested quietly in that pleasant half-sleep of awakening after a good rest. Unconsciously his hand moved down searching for a blanket, but of course there was none. He'd never needed one before.

He unconsciously groped again for the blanket, then opened his eyes wide and sat up. There was a slight chill in the air at that! Now he noticed the monkey, asleep, curled up tightly against his leg. That was odd because previously the little fellow had slept outside. What had brought him in?

Robin got up and Cheeky, as Robin had named his friend, woke up instantly and leaped to his shoulder. "What's the trouble, fellow?" asked Robin, patting him on the head. Then the young man left the cave and looked around. At first nothing seemed greatly changed. The temperature had dropped a few degrees, no doubt about it. Yet there was no special draft, no break in the bubble walls to account for it.

He looked at the plants and then realized that some of them were beginning to change color. A grayness was creeping in subtly. The balls of Moontree fruit, which had been his chief sustenance, were showing signs of wrinkling and had either already shriveled or were beginning to.

Robin glanced around sharply, looked into his notebook calendar. He calculated the days that had passed. When his rocket had crashed the Moon had been full. This meant it was high noon of a Lunar day on the surface above. But a Lunar day lasted about a Terrestrial month—twenty-eight days to be exact. When the sun was at its height, the temperature of the surface crust was to be measured as high as 240 degrees Fahrenheit. By sunset it might be down to 160 degrees, but immediately after sunset it would drop with great speed and shortly begin to go as low as a hundred below zero and continue to drop for yet another hundred degrees.

And Robin had perhaps been in his sublunar cavern for ten or maybe twelve days. The sun had set above, the Lunar night was there. Though the cavern was insulated by the best sort of insulation in the universe—a honeycomb of several miles between the surface and itself—a honeycomb in many cases consisting of sealed bubbles, some near vacuums—there was bound to be a gradual loss of the stored-up heat from the long Lunar day. It might take a while for this to become noticeable, especially in view of the obviously warm volcanic action from the unseen areas near the core of the Moon below, yet there it was.

So now Robin knew that the Lunar day did have a counterpart here, that there would be monthly seasons in his cavern and that he was facing a winter that might last ten days more.

He looked around, pondering this. Could he survive? He had probably only a short time to work this out. Obviously he had to work fast and make good.

"Come, Cheeky," he said, "no time for foolishness. No daily swim in the river this morning. Harvest time is here."

He glanced at his rabbits, but they did not seem to mind the temperature drop. He went into the clumps of Moontrees and began to gather their fruit—the big balls—as fast as possible. They were still plump enough to hold food-pulp. He realized that if he waited, they would probably dry up on their trees, shrivel to seed as the increasing cold drew the moisture from them. He spent that day in gathering a harvest, in piling great masses of the fruit in a small cavern-wall bubble near his sleeping chamber. When he had amassed enough to see him through at least two weeks, he gathered the fallen trunks and dried-up old stalks and piled them in the narrow entrance to this storage cave. He built a fire there, paced it out, and spread it out to burn slowly. He would have to keep this fire going and another like it in front of his own sleeping cavern.

He transferred the rabbits to his bubble-home also, rebuilding their pen.

As he had expected, the temperature in his hidden world continued its fall. A few days later it was quite chilly and the Moontrees had acted as he surmised. Their fruits had withered quite rapidly, finally dropping off as small hard seeds. The tree stalks dried out, turned hard, and fell. Robin gathered them as fuel for his fires, found that they were quite excellent, and also that the fresh-fallen ones could be woven into basketry.

The river continued to flow, but was more sluggish, and its waters began to grow cold. On the other hand, the Moonworms and other little creatures seemed to be having their heyday. They were out in quantities greater than he had ever seen and were busily gathering the fallen seeds, carrying them away, evidently preferring them to the fruit.

Robin made himself a jacket from an extra part of the padding, stitching it together with cord and thongs made from shell fibers. With a fire going at the door of his cave, he found he still wouldn't need blankets.

During the balance of the Lunar night Robin was forced to remain close to his caves, tending his fires, conserving all his energies. Outside, the temperature never actually reached freezing, or at least not that Robin could estimate. But at its worst, it was definitely chilly and the river fairly cold.

The view within the cavern cleared somewhat of its usual mugginess and he could see much more. He could now make out the walls on all sides, and discovered that the farthest distance, in the direction in which the river ran, was perhaps several miles off. The vegetation had mainly flattened, was drying up, and he could see everywhere the little ball-segmented insects humping and squirming about.

He saw a number of varieties he had not noticed before. One day venturing out with his bow and arrow, he disturbed something working amid a pile of broken stalks. The thing rushed out, directly at him. It was large, as large as a dog, and it ran straight for him silently, its wide mouth gaping.

He shot it, saw it fall over as it was about to leap at him. When he dug his arrow out, he saw that it was no wormlike insect, no segmented creature. It was a recognizable animal, a creature with two short stubby feet, two small extensions that were like hands on each side of an oval body. A definite head surmounted this, with one eerie eye set in the middle over its wide mouth. Two little breathing holes in the side attested to its possession of lungs. A long, curving rod grew out of the top of its skull and held a large yellow light-ball over it.

There was yet another peculiarity about this Moonhound, as Robin called it. It had no definite color. Its skin was faintly transparent, and he could see its inner organs shadowy within.

All this reminded Robin that there must be vast cavern worlds totally without light, yet having flora and fauna.

When there is no light, there is no need for pigmentation. Hence, this creature had none.

Robin also surmised that it was probably the cold that drove this beast into the lighted cavern in search of food, for he had never seen evidence of anything that large during the warm period.

Robin brought the carcass back to his cave and went to work to skin and cook it. At first he was not going to, for the hairless, colorless nature of it was rather repellent. But one thing Robin had learned long ago was not to let his emotions dictate to his needs. Like it or not, he was going to make use of everything here he could. He had a task, and that was to survive.

As a matter of fact, the meat cooked very nicely, turned brown in the fire, and tasted good. Further, it had a bone structure, which the Moonworms hadn't, and Robin saved these bones, knowing that there were many things that they could be used for. He remembered museum exhibits of bone needles, bone knives, and bone implements, including arrowheads and buttons that the Indians had made use of.

After that, Robin deliberately hunted for these Moonhounds and caught several others before the winter was over.

The warmth returned about when Robin had figured, starting a day or so after the surface sunrise. It rose rapidly, faster than it had fallen, and just as fast, new Moontrees were shooting up, new Moonmushrooms were growing, and the river was becoming warmer.

As time went on and month followed month, Robin found himself working into a comfortable, if primitive, routine. He charted exactly what to do on what days. He could tell in advance what he would be eating, what he would be harvesting. His rabbits had become sufficient in number to permit slaughtering, and he began to acquire a pile of rabbit furs. He found it no longer possible to keep all his rabbits in one pen, and finally liberated the majority of them and left them to shift for themselves. This worked out fine, and he never lacked the sight of at least one or two bunnies anywhere he looked. During the two weeks of winter each month they simply holed up as they might have done on Earth. It was an odd sight seeing the rabbits run wild, for their powerful leg muscles were many times stronger than was required by the weak gravity and, when they ran in a hurry, they would bounce many feet high in fantastic leaps.

Robin was now wearing a rabbit-fur outfit of coat, pants, and hat during the winter periods, equipped with bone buttons he'd carved from the Moonhound skeletons. He was, if anything, beginning to gain weight, but he was also aware of the paleness of his skin. He wondered whether staying in this sunless world a sufficient number of years would not make him as palely transparent as the Moonhounds.

But all this time Robin had not forgotten his ultimate mission—to reach the surface and signal for help. He had worked out the problem in his own mind. He had to make some sort of space suit, something that would permit him to venture out on the nearly airless surface long enough to set up a signal that astronomers might see.

He knew he had the materials for part of this suit in the metal salvaged from the rocket nose. He could polish a section sufficiently to make a heliograph with which he could flash a code message to any high-powered telescope that might be pointed his way. But he had also to fashion the metal into an airtight space helmet, and that he did not know how to do. The suit itself he could probably fashion from cloth and tanned skins, sew and seal it tight enough with animal fats and bone glue to be airtight for a short period, but he needed the helmet. He had the glass for it too, the little peepholes for the camera outlets and a large circular plate that had been set in the very base of the cargo nose and evidently intended for a wide-vision camera shot of the Earth. This plate would be his face plate.

Robin was aware of the hissing noise that he had first noticed on his arrival, but he had never investigated it. It was far off, somewhere along the wall of the cavern. One work period, when he found himself ahead of schedule, he set out to find the source of the noise.

Following the wall, with Cheeky running ahead chattering, the hiss gradually grew in volume. Robin made his way over a sharp cleft, skirted a large bubble-cave in the wall, and after about two miles of walking, came upon the source.

Issuing from a break in the outer cavern wall was a stream of blue flame. For several hundred feet around it no vegetation grew, the ground being covered with thin gray ash. Robin looked at the loudly hissing lance of blue fire.

It probably was a breakthrough from some adjoining bubble, one filled with a gas of some inflammable sort. Somehow in the course of the breakthrough, this leakage had been set aflame. And there it was now, a burning gas jet, sharp and hot.

At that moment, Robin knew he had the answer to his metalworking problem. He'd tried to melt the metal of the rocket over his fires but he had been totally unsuccessful. But this jet, this hot blue flame, this surely would do the trick!

For him the space helmet was now a certainty. It might take time, but now it could be done. That and more was possible, for he had enough metal to make a few necessities like a decent frying pan and a pot to use for boiling and perhaps a water container for a really long exploration trip.

That was the end of Robin's first "Stone Age" period and the beginning of his "Iron Age."