More than a little reminiscent of The Little Prince, with a touch of Louis Slobodkin's classic Space Ship Under the Apple Tree, this is a dialogue between a visitor from space and an eight-year-old boy waiting at home while his mother is in the hospital giving birth; it links informal ink drawings with a series of ingenuous philosophical ruminations and explanations of natural processes. Joe is alone when a flash of light leaves Mika, a diminutive Mumbo from planet Eljo, hanging by his heels in the apple tree. Very like a toddler in appearance, clothing (pajamas), and habit of sucking his thumb when thinking, Mika views the world with wide-eyed wonder, asking deceptively simple questions (""If [a cat] can't talk, does that mean it can't think?""), learning from Joe about dinosaurs, fish, and the history of life on Earth, teaching him that nothing is ""ordinary,"" and that there are great forces at work in the universe. The illustrator places small human, alien, and animal figures on the surfaces of tiny worlds, or within and around lines of text; Mika and Joe sit cozily together on rock and rooftop, until Mika is replaced at the end by Michael, Joe's new brother. Gaarder (Sophie's World, 1994) keeps the tone light, but brings up plenty of worthwhile, often knotty ideas.