"Oh?" said Jerry, halting. "What's that, Ensign?"

"How'd the initial roborocket miss the thing and its kind when it circled the planet before colonization began?"

"That's a moot question," said Jerry. "But my conjecture is that the scanner always caught it when it was assuming some other form. Since its victims were always indigenous to this planet, the things familiar to them were also of this planet, and the scanner-beam couldn't detect any life-pulses which were dissimilar to already-known species."

"I'll be damned," said Bob. "It's almost childishly simple when you explain it." Then, as Jerry went to start off again, Bob stopped him with an exclamation.

"What about that melting mine car I read about on the translation sheets? Was that for real, or wasn't it?"

Jerry shook his head. "Part of the general mimetic illusion, like the motionless clouds and unmoving trees. It let me see what I expected to see. In reality, I was just in the woods near the mine area, where you came upon the creature to destroy it." Jerry started slowly moving away once more.

A few steps further, and Bob halted again. "One final point, sir. That life-pulsing reading of point-nine-nine-nine. If the thing's pulsation was that powerful, I should think it would've been a lot harder to knock off than it was."

"You're right," said Jerry. "It would have been. But its life-pulse wasn't nearly that high."

"But the scanner-beam—" Bob protested. "When the colony sent up that roborocket, after those miners vanished, it reported an unknown life-pulse of point-nine-nine-nine. If that wasn't the alien's life-pulse, what the devil was it?"

Jerry patted Bob on the shoulder. "You're forgetting the mimicry. The roborocket they sent up caught the alien off-guard, in its own shape, not imitating some other life-form's pulsations. It detected the beam, since a scanner picks up mental pulses, and it instantly assumed the life-pulse of a creature it assumed no roborocket would worry about."

"What? What life-pulse, sir? What kind of life?"

"Atomic life, Ensign," said Jerry. "That bright green blip you and I studied so assiduously was the life-pulse of an atom-powered creature. It was another roborocket."

And as Bob stared after 

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