Children, as harried parents often complain, have...

Children, as harried parents often complain, have an insatiable curiosity about the world. They never seem to tire of asking questions and are particularly adept at posing ones that adults have trouble answering. Why is the sky blue? Why are girls different from boys?

Children, parents also frequently observe, have vivid imaginations. They can tell the most astounding lies with the most wide-eyed expressions of innocence. Children’s love of exaggeration is part of the same process as their quest for facts. It is one of the ways they test the limits of their world, explore the boundaries of the possible. The separation between fact and fantasy, the real and the imaginary, is a common thread in several new children’s books.

In There’s a Hippo in My Bath! (Doubleday: $12.95; unpaginated) Kyoko Matsuoka uses a child’s power of imagination to transform a bath into magical adventure. Children often have fears about bath time. One of Mister Rogers’ more memorable songs addresses one of them: “You can never go down, never go down, never go down the drain.” Matsuoka’s delightful picture book speaks to another concern of the very young--what may be lurking in the bathwater.

A mother leaves her son alone to take a bath. The boy climbs into the tub with his rubber duckie. The duck dives to the bottom of the bath and surfaces to report company. A sea turtle pops his head out of the water. The duck protests that the tub isn’t an ocean, but that doesn’t prevent a pair of penguins from bobbing up next. They are followed by a soap-eating seal, a hippopotamus, and finally a gigantic whale. They all cavort in the bathtub until the boy’s mother comes to investigate. The animals immediately vanish. The boy wisely decides to keep his bathmates a secret.

Matsuoka uses exaggeration the way children often do--to show their mastery. You can only kid about creatures swimming in your tub if you know the idea is impossible. Akiko Hayashi’s golden-hued drawings playfully convey the steamy, shimmering bathroom and the benevolent animals who emerge.

Samuel Marshak’s funny poem, The Pup Grew Up!, translated by Richard Pevear and illustrated by Vladimir Radunsky (Henry Holt: $13.95; unpaginated), uses exaggeration in another way familiar to children. It is the kind of tall tale a child might tell when forced to explain an occurrence he can’t account for. The story concerns a lady who boards a train with a tiny Pekingese and gets off with a Great Dane. The book’s title is the explanation officials give for the discrepancy. Adults will appreciate Marshak’s sly dig at bureaucracy, which is still as fresh as it was when the book was first published in the Soviet Union in 1926. Children will delight in Marshak’s sense of absurdity. Vladimir Radunsky’s illustrations catch Marshak’s tone perfectly. Radunsky’s exaggerated people, bold colors, and graphic use of space reinforce the lunacy of this tale, written by Russia’s best-known children’s author.

Stephanie Waxman’s What Is a Girl? What Is a Boy? (Thomas Y. Crowell: $10.95; unpaginated) is a different kind of picture book for 3- to 7-year-olds, a volume of straightforward text and photographs that address the basic biological differences between the sexes. As Waxman writes in her foreword, “Enlightened adults are apt to play down the difference between the sexes and encourage the similarities in an effort to foster equality and non-sexist attitudes. Yet in doing so, we may be disregarding the child’s need for elementary information.”

Children are naturally curious about sexuality. Since many adults are uncomfortable discussing the subject, though, they often don’t give children the straight facts they want to know. As a result children are left to their own imaginations (and society’s sexist stereotypes) to explain the differences between the sexes: “Boys are ugly!” “Girls are dumb!” “Boys don’t cry!” “Girls aren’t strong!”

Waxman begins by raising and then discarding some of these familiar stereotypes: “Some people say a girl is someone who plays with dolls. But Noah is taking care of his doll. And he’s a boy.” She then goes on, in a matter-of-fact and non-threatening way, to present the simple biological distinction between the sexes. “A boy is someone with a penis and testicles. A girl is someone with a vulva and vagina.” The photographs show naked children, and a naked man and woman, all unself-conscious and comfortable in their nudity. The book won’t end children’s sexual curiosity, but it can help diffuse the embarrassment that surrounds the discussions of sexuality. By providing the simple facts that children desire, it may even lay the subject to rest for a while.

The Magic School Bus: Inside the Human Body (Scholastic Inc: $13.95; unpaginated) is also about biology. Written by Joanna Cole and illustrated by Bruce Degen, the book is the third of their very popular Magic School Bus science series. Cole and Degen combine fact and fantasy to provide a clever, comic-book style approach to science education. Ms. Frizzle and her class go on a field trip to the science museum to see an exhibit about the human body. On the way (stop me if you’ve seen this at your local movie theater!), the magic school bus is mysteriously shrunk and accidentally swallowed. The field trip becomes a wacky voyage inside the human body.

Cole and Degen borrow from comic books, sci-fi movies, and Saturday-morning cartoons to create a hip pastiche of information and humor. Like sitcom regulars, Ms. Frizzle’s class keeps up a steady patter of jokes. “You mean this body thinks we’re food?” “That’s better than being waste.” It’s humor-coated facts for the TV generation who, supposedly, can’t concentrate longer than the span between commercials and are incapable of digesting their information straight. Whether or not that unhappy assumption is true, I found the book long on humor and short on science. The comic-book style of joining pictures with text in boxes and balloons also makes some of the anatomical drawings, especially the workings of the lungs, difficult to read. Despite these objections, I’m sure many children will enjoy this. But I think the book is more useful at provoking curiosity about the body than explaining how it works. Those already curious may find this book as unnourishing as Saturday-morning cartoons.