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didn't she asked, "What about it? Do you want to go?"
    "Now, wait a damn minute," Gorshak said, scowling. "I thought we were going to have a little private party right here. Just family." He reached over to pat Dekker's knee and didn't notice when Dekker, who didn't consider Tinker Gorshak any part of his family, pulled stiffly back. Gorshak went on, "What's the use of him getting mixed up with those people? All they want to do is suck our blood. I think he should tell this girl to take her invitation and stuff it."
    "Tinker. It isn't going to hurt him to see how the other half lives," Gertrud said, "if he wants to go. What about it, Dekker?"
    Dekker said, "I probably ought to go, I think. I mean, it's kind of like the Law of the Raft, isn't it?"
    Tinker scowled, then grinned. To Gerti he said, "It's something we were talking about. Maybe he's right."
    "All right, then," Gerti DeWoe said, not bothering to try to understand the allusion. "Then what about a present?"
    Another surprise. Dekker gave his mother a puzzled look. "A present?"
    "That's right, a present. Earthies are always giving presents to each other. You can't go to a party without bringing something along to give your hostess."
    "But I don't have anything to give away," Dekker protested.
    Tinker Gorshak said sourly, "Who does? But that doesn't stop the mudsuckers from always wanting something anyway." He thought for a moment, then shrugged. "Well, if you're sure you want to do this—Tell you what, Dek. Let me make a couple of calls and see what I can do for you.

9
     
     
    A good-sized comet can mass more than a hundred billion tons, and in the richest comets as much as four-fifths of that mass may be water—not liquid water, of course, but ice is just as good.
    Water's the first thing you want to grow crops. Old Earthie farmers used to calculate that it took about seven million tons of irrigation water to lay one inch—in modern measures, two and a half centimeters—of water on a hundred square miles ("miles"!) of farmland. The other figure to remember is that you need about ten "inches" a year to make most crops grow, even with trickle irrigation.
    What that adds up to is that you could wet down nearly a million and a half square miles with one decent-sized comet, sort of.
    In practice it's harder than that. You can make those sums work only if you could keep the water contained in one place—which you can't. And also if most of it didn't get blasted right back into space by the impact—which it does. And if that was all you wanted . . .
    But of course the Martians wanted much more than that. They wanted water to keep, and air besides. So they needed a lot of comets, because what they wanted was to make the whole old planet bloom.
    What a lucky thing it was for all of them that the Oort had literally trillions of the things.

1O
     
     
    When Dekker DeWoe, gift in one sweaty hand, turned up at the home of the Cauchy family, the first thing that surprised him was the walls.
    Dekker knew what walls were supposed to look like. All Martians lived within walls all their lives, and all the walls in Sunpoint City, as in every Martian settlement, were made out of the same material. The stuff looked like poured concrete but, since there certainly wasn't enough spare water on Mars to waste on such water-intensive processes as pouring concrete, certainly wasn't. What the walls were made of was the cheapest and most convenient material available on Mars: rock. Rock was cheap to obtain and easy to deal with—as long as you had plenty of solar heat. You just dug up some of the bare rock that lay all around the surface, and then you crushed it, pressed it, and heated it until it sintered into flat construction panels.
    Of course, most Martians then did as much as they could to decorate those monotonous walls. They painted them at least—"crayoned" them might have been a better word, because the pigment was in waxy sticks that you just rubbed on. Or they hung pictures