 Why does the narrator lie to his son?
Yet I had always been so sure I could create them that I had been calling them volplas for ten years. 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. 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. Outside this little town, I found a litter of flat stones that had young black-snakes under each slab.
 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.