"And Kemplerer rosettes," said Louis. A barely audible hum told him that the ship was moving. He and Speaker stowed their luggage and joined the others in the couches. Teela handed Louis a red, fruity drink in a squeezebulb.

"How much time have we got?" he asked the puppeteer.

"An hour until we land. Then you will be briefed on our final destination."

"That should be long enough. Okay, speak to us. Why flying worlds? Somehow it doesn't seem safe to throw habitable worlds about with such gay abandon."

"Oh, but it is, Louis!" The puppeteer was terribly earnest. "Much safer than this craft, for instance; and this craft is very safe compared to most human-designed craft. We have had much practice in the moving of worlds."

"Practice! How did that happen?"

"To explain this, I must speak of heat … and of population control. You will not be embarrassed or offended?"

They signified negative. Louis had the grace not to laugh; Teela laughed.

"What you must know is that population control is very difficult for us. There are only two ways for one of us to avoid becoming a parent. One is major surgery. The other is total abstinence from sexual congress."

Teela was shocked. "But that's terrible!"

"It is a handicap. Do not misunderstand me. Surgery is not a substitute for abstinence; it is to enforce abstinence. Today such surgery can be reversed; in the past it was impossible. Few of my species will willingly undergo such surgery."

Louis whistled. "I should think so. So your population control depends on will power?"

"Yes. Abstinence has unpleasant side effects, with us as with most species. The result has traditionally been overpopulation. Half a million years ago we were half a trillion in human numbering. In kzinti numbering -"

"My mathematics is good," said the kzin. "But these problems do not seem to relate to the unusual nature of your fleet." He was not complaining, merely commenting. From the refreshment console Speaker had procured a double-handed flagon of kzinti design and half a gallon's capacity.

"But it does relate, Speaker. Half a trillion civilized beings produce a good deal of heat as a byproduct of their civilization."

"Were you civilized so long ago?"

"Certainly. What barbarian culture would support so large a population? We had long since run out of farming land, and had been forced to terraform two worlds of our system for agriculture. For this it was necessary to move them closer to our sun. You understand?"

"Your first experience in moving worlds. You used robot ships, of course."

"Of course … After that, food was not a problem. Living space was not a problem. We built high even then, and we like each other's company."

"Herd instinct, I'll bet. Is that why this ship smells like a herd of puppeteers?"

"Yes, Louis. It is reassuring to us to smell the presence of our own kind. Our sole and only problem, at the time of which I speak, was heat."

"Heat?"

"Heat is produced as a waste product of civilization."

"I fail to understand," said Speaker-To-Animals.

Louis, who as a flatlander understood perfectly, forebore to comment. (Earth was far more crowded than Kzin.)

"An example. You would wish a light source at night, would you not, Speaker? Without a light source you must sleep, whether or not you have better things to do."

"This is elementary."

"Assume that your light source is perfect, that is, it gives off radiation only in the spectra visible to kzinti. Nonetheless, all light which does not escape through the window will be absorbed by walls and furniture. It will become randomized heat.

"Another example. Earth produces too little natural fresh water for its eighteen billions. Salt water must be distilled through fusion. This produces heat. But our world, so much more crowded, would die in a day without the distilling plants.

"A third example. Transportation involving changes in velocity always produces heat. Spacecraft filled with grain from the agricultural worlds produce heat on reentry and distribute it through our atmosphere. They produce more heat on takeoff."

"But cooling systems -"

"Most kinds of cooling systems only pump heat around, and produce more heat for power."

"U-u-urr. I begin to understand. The more puppeteers, the more heat is produced."

"Do you understand, then, that the heat of our civilization was making our world uninhabitable?"

Smog, thought Louis Wu. Internal combustion engines. Fission bombs and fusion rockets in the atmosphere. Industrial garbage in the lakes and oceans. It's often enough that we've half-killed ourselves in our own waste products. Without the Fertility Board, would the Earth be dying now in its own waste heat?

"Incredible," said Speaker-To-Animals. "Why didn't you leave?"

"Who would trust his life to the many deaths of space? Only such a one as me. Should we settle worlds with our insane?"

"Send cargos of frozen fertilized ova. Run the ships with crews of the insane."

"Discussions of sex make me uncomfortable. Our biology is not adapted to such methods, but doubtless we could evolve something analogous … but to what purpose? Our population would be the same, and our world would still have been dying of its own waste heat."

Irrelevantly, Teela said, "I wish we could see out."

The puppeteer was astounded. "Are you sure? Are you not subject to the fear of falling?"

"On a puppeteer ship?"

"Ye-es. In any case, our watching cannot increase the danger. Very well." Nessus spoke musically in his own tongue, and the ship vanished.

They could see themselves and each other; they could see four crash couches resting on emptiness, and the refreshment console in the middle. All else was black space. But five worlds glowed in white splendor behind Teela's dark hair.

They were of equal size: perhaps twice the angular diameter of the full Moon as seen from Earth. They formed a pentagram. Four of the worlds were circled by strings of tiny, glaring lights: orbital suns giving off artificial yellow-white sunlight. These four were alike in brightness and appearance: misty blue spheres, their continental outlines invisible at this distance. But the fifth …

The fifth world had no orbital lights. It glowed by its own light, in patches the shapes of continents and the colors of sunlight. Between the patches was a black that matched the black of surrounding space; and this black, too, was filled with stars. The black of space seemed to encroach on continents of sunlight.

"I've never seen anything so beautiful," said Teela, with tears in her voice. And Louis, who had seen many things, was inclined to agree.

"Incredible," said Speaker-To-Animals. "I hardly dared believe it. You took your worlds with you."

"Puppeteers don't trust spacecraft," Louis said absently. There was a touch of cold in the thought that he might have missed this; that the puppeteer might have chosen someone in his place. He might have died without seeing the puppeteer rosette …

"But how?"

"I had explained," said Nessus, "that our civilization was dying in its own waste heat. Total conversion of energy had rid us of all waste products of civilization, save that one. We had no choice but to move our world outward from its primary."

"Was that not dangerous?"

"Very. Then was much madness that year. For that reason it is famous in our history. But we had purchased a reactionless, inertialess drive from the Outsiders. You may guess their price. We are still paying in installments. We had moved two agricultural worlds; we had experimented with other, useless worlds of our system, using the Outsider drive.

"In any case, we did it. We moved our world.

"In later millenia our numbers reached a full trillion. The dearth of natural sunlight had made it necessary to light our streets during the day, producing more heat. Our sun was misbehaving.