As I trailed Clatclit up the wearisome slope that was taking us to the surface, I did some heavy thinking. The Ancients, before Earthmen first landed on Mars, probably had wandered about the planet freely, on the surface, living in their dwellings of parabolite, using their artifacts of the same impervious mineral. Then Earth, that paradoxically peace-loving and war-making planet, lands colonists. The Ancients, just from plain discretion, hide themselves and observe these unwelcome newcomers. Once it becomes clear to them that there is a potential menace from Baxter--who is no young chicken, having been in power before the first landing--they stay hidden, and start scheming to get rid of this guy who can jolt them out of their liaison. I pondered over that bit. The Ancient had said that Baxter intended doing it by detonating a portion of their contact-material. Hell, they must mean parabolite! What other substance in the solar system was so alien to-- And with that thought, I suddenly knew the secret of that apparently impervious mineral's strength. No wonder it could not be destroyed! It was only in existence in our skimpy three dimensions in a fractional way. One-fourth of it was always present in the Ancients' world, since it couldn't _fit_ into our universe in its entirety. And that meant not only one-fourth of its apparent mass, but one-fourth of even its atomic structure! Even the collapsers, working on subatomic particles, were at a disadvantage. You can't nudge an electron out of orbit if it isn't actually fixed in that orbit. Three-quarters of those four-dimensional electrons were always cushioned by that elusive final segment that lay outside our universe. So trying to destroy parabolite by force was in the same class as trying to shatter a rubber ball with a hammer; a rubber ball which was hanging from an elastic cord, in fact! It just gave into the other dimension and rebounded frisky as ever. "Boy," I thought, "this is going to put the skids under that scientific theory about parabolite's imperviousness. Parabolic molecules, ha! Well, it was a good theory while it lasted; it fit the known facts, at least. Hell, the stuff even has the wrong name! It ought to be called Elasto-plast, or some such euphonic label." Clatclit paused in his climb up the tunnel slope, and turned a querying stare on me. "Was I talking aloud?" I asked. Curious nod. "Sorry, it's nothing," I said, indicating that he should proceed with our journey. "Just the salesman in me coming to life. You can't have public interest without catchy trade-names. Once an ad man, always an ad man." Clatclit looked positively bewildered. "Sorry. Business talk," I explained. He shrugged and continued his upward climb, with me tagging after the bobbing pink taillight. * * * * *