Children think grown-ups know everything there is to know, and adults like to keep it that way. When the tot closest to you decides to play Stump the Dad, you can earn extra credit points by consulting How Come? Every Kid's Science Questions Explained, by Kathy Wollard (Workman, $9.95). Then you won't have to fake it through questions such as "How did the sun form?" and "Why are dinosaurs extinct?"