When the twins reached their own room Castor got down the general catalog of Four Planets Export. Pollux said, "Cas?"

"Don't bother me."

"Have you ever noticed that Dad always gets pushed around until he gets his own way?"

"Sure. Hand me that slide rule."

The Rolling Stone was moved over to the spaceport by the port's handling & spotting crew - over the protests of the twins, who wanted to rent a tractor and dolly and do it themselves. They offered to do so at half price, said price to be applied against freightage on their trade goods to Mars.

"Insurance?" inquired their father.

"Well, not exactly," Pol answered.

"W'e'd carry our own risk," added Castor. "After all, we've got assets to cover it."

But Roger Stone was not to be talked into it; he preferred, not unreasonably, to have the ticklish job done by bonded professionals. A spaceship on the ground is about as helpless and unwieldly as a beached whale. Sitting on her tail fins with her bow pointed at the sky and with her gyros dead a ship's precarious balance is protected by her lateral jacks, slanting down in three directions. To drag her to a new position requires those jacks to be raised clear of the ground, leaving the ship ready to topple, vulnerable to any jar. The Rolling Stone had to be moved thus through a pass in the hills to the port ten miles away. First she was jacked higher until her fins were two feet off the ground, then a broad dolly was backed under her; to this she was clamped. The bottom handler ran the tractor; the top handler took position in the control room. With his eyes on a bubble level, his helmet hooked by wire phone to his mate, he nursed a control stick which let him keep the ship upright. A hydraulic mercury capsule was under each fin of the ship; by tilting the stick the top handler could force pressure into any capsule to offset any slight irregularity in the road.

The twins followed the top handler up to his station. "Looks easy," remarked Pol while the handler tested his gear with the jack still down.

"It is easy," agreed the handler, "provided you can out-guess the old girl and do the opposite of what she does - only do it first. Get out now; we're ready to start."

"Look, Mister," said Castor, we want to learn how. We'll hold still and keep quiet."

"Not even strapped down - you might twitch an eyebrow and throw me half a degree off."

"Well, for the love of Pete!" complained Pollux. "Whose ship do you think this is?"

"Mine, for the time being," the man answered without rancor. "Now do you prefer to climb down, or simply be kicked clear of the ladder?"

The twins climbed out and clear, reluctantly but promptly. The Rolling Stone, designed for the meteoric speeds of open space, took off for the spaceport at a lively two miles an hour. It took most of a Greenwich day to get her there. There was a bad time in the pass when a slight moonquake set her to rock­ing, but the top handler had kept her jacks lowered as far as the terrain permitted. She bounced once on number-two jack, then he caught her and she resumed her stately progress.

Seeing this, Pollux admitted to Castor that he was glad they had not gotten the contract. He was beginning to realise that this was an estoric skill, like glassblowing or chipping flint arrowheads. He recalled stories of the Big Quake of '31 when nine ships had toppled.

No more temblors were experienced save for the microscopic shivers Luna continually experiences under the massive tidal strains of her eighty-times-heavier cousin Terra. The Rolling Stone rested at last on a launching flat on the east side of Ley­port, her jet pointed down into splash baffles. Fuel bricks, water, and food, and she was ready to go - anywhere.

The mythical average man needs three and a half pounds of food each day, four pounds of water (for drinking, not washing), and thirty-four pounds of air. By the orbit most economi­cal of fuel, the trip to Mars from the Earth-Moon system takes thirty-seven weeks. Thus it would appear that the seven rolling Stones would require some seventy-five thousand pounds of consumable supplies for the trip, or about a ton a week.

Fortunately the truth was brighter or they would never have raised ground. Air and water in a space ship can be used over and over again with suitable refreshing, just as they can be on a planet. Uncounted trillions of animals for uncounted millions of years have breathed the air of Terra and drunk of her streams, yet air of Earth is still fresh and her rivers still run full. The Sun sucks clouds up from the ocean brine and drops it as sweet rain; the plants swarming over the cool green hills and lovely plains of Earth take the carbon dioxide of animal exhalation from the winds and convert it into carbohydrates, replacing it with fresh oxygen.

With suitable engineering a spaceship can be made to behave in the same way.

Water is distilled; with a universe of vacuum around the ship, low-temperature, low-pressure distillation is cheap and easy. Water is no problem - or, rather, shortage of water is no problem. The trick is to get rid of excess, for the human body creates water as one of the by-prodncts of its metabolism, in 'burning' the hydrogen in food. Carbon dioxide can be replaced by oxygen through 'soilless' gardening' - hydroponics. Short-jump ships, such as the Earth-Moon shuttles, do not have such equipment, any more than a bicycle has staterooms or a galley, but the Rolling Stone, being a deep-space vessel, was equipped to do these things.

Instead of forty-one and a half pounds of supplies per person per day the Rolling Stone could get along with two; as a margin of safety and for luxury she carried about three, or a total of about eight tons, which included personal belongings. They would grow their own vegetables en route; most foods carried along would be dehydrated. Meade wanted them to carry shell eggs, but she was overruled both by the laws of physics and her mother - dried eggs weigh so very much less.

Baggage included a tossed salad of books as well as hundreds of the more usual flim spools. The entire family, save the twins tended to be old-fashioned about books; they liked books with covers, volumes one could hold in the lap. Film spools were not quite the same.

Roger Stone required his sons to submit lists of what they proposed to carry to Mars for trade. The first list thus submit­ted caused him to call them into conference. "Castor, would you mind explaining this proposed manifest to me?"

"Huh? What is there to explain? Pol wrote it up. I thought it was clear enough."

"I'm afraid it's entirely too clear. Why all this copper tubing?"

"Well, we picked it up as scrap. Always a good market for copper on Mass."

"You mean you've already bought it?"

"Oh, no. We just put down a little to hold it."

"Same for the valves and fittings I suppose?"

"Yes, sir."

"That's good. Now these other items - cane sugar, wheat, dehydrated potatoes, polished rice. How about those?"

Pollux answered. "Cas thought we ought to buy hardware; I favored foodstuffs. So compromised."

"Why did you pick the foods you did?"

"Well, they're all things they grow in the city's air-conditioning tanks, so they're cheap. No Earth imports on the list, you noticed."

"I noticed."

"But most of the stuff we raise here carries too high a per­centage of water. You wouldn't want to carry cucumbers to Mars, would you?"

"I don't want to carry anything to Mars; I'm just going for the ride." Mr. Stone put down the cargo list, picked up another. "Take a look at this."