I like the story of the little boy who came to his father and asked him, "Dad, who made God?" The father, engrossed in the evening paper, responded, "Beats me, son." The little boy would not be put off. "Dad, why is the earth round?" The dad answered, "I don't know, son." The boy played for a minute, then asked, "Dad, is there life on other planets?" The father patiently answered, "Nobody knows the answer to that." Finally, the boy asked his father, "Dad, do you mind me asking you all these questions?" The father put down his paper, "Why, not at all, son," he said, "How else are you going to learn?"

Another version:

One father was trying to read the paper. He was asked by his little daughter a whole series of questions-difficult questions which children ask: "Where did God live before he created the earth? What makes the leaves green? Where do butterflies get their wings?" At the end of each question, the father would simply say, "I don't know." Finally the little girl said, "Am I bothering you with my questions?" The father smiled, "Not at all, how are you going to learn if you never ask!"