"I'm coming to that," said Jerry. "The size was off because the first thing I saw when I blinked open my eyes was a distant copse of trees, which I took to be an upright pile of leafy twigs. Since my mind possessed information regarding the relative size of babies and twigs, the alien immediately made sure my mind saw other things in the same perspective. By the time it realized it had made an error, it was too late to normalize the baby's dimensions; that would have given its fakery away."
"But why did the thing choose a baby?"
"Because that was the thing's protection! It had a powerful hypnotic power, one that worked on its victims' minds directly through its telepathic interference with sensory perception. It always appeared as the thing the victim would be least likely to harm. In my case, a baby. But it made a slight error there, too. I'm a bachelor, Jana. There's only one baby with whom I ever had any great amount of experience: myself."
"And the invisibility?"
"I have no recollection, even now, of my body when I was a baby. I may have stared at my toes, played with my fingers, but they just never registered on my consciousness as being part of myself. So the thing was stuck when it came to reproducing me visually, since it depended upon my own memory for details. But it was able to supply the way I'd felt as a baby. Every baby has an acute awareness of its own skin; it will cry if any particle of its flesh is bothered in the slightest. So the alien fed the 'feel' of my baby-body back to me, if not the view. Which is why the electronic brain on the ship was able to duplicate the detail into an almost perfect replica of my babyhood likeness."
Jana nodded, as she finally understood the meaning of that strange illusion. "And this time? That post-hypnotic suggestion you had the doctor give you, I mean: that you'd think you were a gray rat until such time as the light of the arc-torch caught you directly in the eyes...."
"Duplicity, Jana. It had to be that way. The alien was very sure of its powers. If I returned, and it were a baby again, I couldn't attack it or thwart its ends. And such an attack was necessary. I had to be able to fight it, to hold it in place for that last moment before it was destroyed. Which is why I chose a gray rat, an animal I cannot bear the sight of. When the light struck my eyes and I became myself again, I caught the alien unawares. Then, before it could change to a baby, and start lulling me back into camaraderie, it was too late. Bob had given the order to fire. And here I am."
Hurrying footsteps sounded in the corridor. The door burst open and Bob rushed in, his face anxious and creased with worry until he saw Jerry sitting on the couch, alive and well.