 Why does the narrator lie to his son?
It would be several hours before they would begin to move, to learn to feed and play, perhaps to learn to fly. 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. He would conclude, "I am convinced that they have a language and speak it intelligently." Reporters would "expose the truth" and ask, "Where have these aliens come from?" Volpla wisdom would become a cult—and of all forms of comedy, cults, I think, are the funniest. A 'copter sidled over the ridge, then came just above the oak woods toward us.
 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.