 Why does the narrator lie to his son?
I heard my daughter's running feet in the animal rooms and her rollerskates banging at her side. Yet I had always been so sure I could create them that I had been calling them volplas for ten years. No, twelve. I had called them volplas since the day old Nijinsky's elongated arms and his cousin's lateral skin folds had given me the idea of a flying mutant. "Tell your mother that I retaliate. Again in the laboratory, I entered the metabolic accelerator and withdrew the intravenous needles from my first volplas. The accelerator had forced them almost to adulthood in less than a month. It would be several hours before they would begin to move, to learn to feed and play, perhaps to learn to fly. Meanwhile, it was clear that here was no war of dominant mutations. I flipped a hamburger and a slice of onion onto a plate and picked up the ketchup and said, "I've reached the dangerous age." I wiped the back of my hand across my mouth and said aloud, "Yes, sir, the dangerous age. I kissed her until I heard rollerskates coming across the terrace from one direction and a horse galloping toward the terrace from the other direction. Outside this little town, I found a litter of flat stones that had young black-snakes under each slab. Except for the face, chest and belly, they were covered with a soft, almost golden down. The cranium was in the same proportion to the body as it is in the human. The spars had been common to the basic colony for years and were the result of serial mutations effecting those greatly elongated fifth fingers that had first appeared in Nijinsky. No longer jointed like a finger, the spar turned backward sharply and ran alongside the wrist almost to the elbow. As they swept out and forward, the lateral skin that had, till now, hung in resting folds was tightened in a golden plane that stretched from the tip of the spar to his waist and continued four inches wide down his legs to where it anchored at the little toe. By four o'clock that afternoon, I was feeding them solid food and, with the spars closed, they were holding little cups and drinking water from them in a most humanlike way. I would teach them legends: that they had come from the stars, that they had subsequently watched the first red men and then the first white men enter these hills. There would be volpla colonies all up and down the Coast before anyone suspected.
 For his joke to have its desired effect, no one can know the full extent of his experiment.
 Even though his son is a young man sooner than already, he is still too young to learn the full scope of the truth.  A joke stops working when someone attempts to explain it.  For his joke to have its desired effect, no one can know the full extent of his experiment.  He is an eccentric and must abide by his personal eccentricities.