Old and young meet for an exchange of life's lessons in this latest offering from Grindley (The Big Crocodile Book, 1993, etc.). Rabbit and Donkey live in the same field, but in different worlds. Donkey is old, wise, and slow; Rabbit is everywhere, and bristling with questions. Donkey is happy to explain it all if Rabbit will sit still. Rabbit's patient attendance, however, is more easily promised than accomplished, and his joy in movement is apparent in Varley's blithe watercolors, which rely heavily on cartoony whiz lines to limn action. When one day Rabbit erupts from the lesson and stays away long enough to arouse concern, Donkey seeks him out, making new discoveries along the way. Rabbit, stuck in the middle of a bush, shows Donkey things he's never seen before, and comes up with an answer to the question of the title: The sky is blue ""because that was the only color left in the paint box."" Donkey's amusement at this unleashes his own exuberance, and his behavior is more typical of Rabbit's than his own stately ways. A wry and gentle book of friendship and discovery.