at the stupidity of this one in attacking you when your husband and I were in the next room—and they are really not especially dangerous provided you know the defense against them.

"You see, their body structure, while based on the same elements as our own, involves large quantities of free hydrogen between the body cells. Hydrogen ignites in ordinary air with explosive force—the end product's water—and when I threw that burning stuff at the creature, the hydrogen in its tissues exploded. It blew up. There's probably a good deal more water vapor in the air in this room than there was before I got rid of the thing."

Kauss cleared his throat.

"There's another life form," he said with a faintly professional air, "allied to the Mocker, but with important differences, which is far more dangerous. That's the Stroller."

"The Stroller?" Marta asked. George had put his arm around her; they were not an affectionate couple, but the moment seemed to call for tender demonstration. "Why do they call it that?"

"No one knows, exactly. It seems to come from the creature's own name for itself, for its fondness for taking long, long, walks."

Kauss turned the cigar in his mouth. He poked at the suit lying on the floor with the toe of his shoe.

"What does it do?" Marta queried. "Why is it so terribly dangerous?"

"The Stroller doesn't hunt a host, like the Mocker," Kauss replied. "Early in life it takes over the identity of some human being, and it remains indistinguishable from a human being to any usual test. It's so dangerous because there's absolutely no defense against it. No free hydrogen in its tissues. It's indestructible."

"My!" Marta said. "Goodness!"

"It feeds, like the Mocker, on both the flesh and the life force of human beings. Fortunately"—Kauss smiled—"it's very, very rare. There are probably only a few Strollers in the entire solar system, and they reproduce only at widely separated intervals."

Once more Kauss halted and poked absently at the clothing on the floor with the toe of his boot.

"There's a peculiarity about their feeding habits," he said. "They'll go for years without feeling any desire to eat their special food, and then something will happen which makes them—greedy, and after that they can't be stopped before they feed."

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