An Oasis of Their Own : THE COOLEST PLACE IN TOWN <i> by Kathy Caple (Houghton Mifflin: $13.95; 32 pp.) </i>

To Hank and Zoey, two hippo brothers desperate for relief on a swelteringly hot day, their old plastic swimming pool is a natural, and indeed, “The Coolest Place in Town.” The only problem is their big sister Dory thinks so too. She tricks her gullible brothers into running inside the house for some nonexistent fruit punch; when they realize they’ve been duped, they come charging back to find Dory basking in the cool water--and she refuses to budge (“You can have it all winter,” Dory taunts them). Now Hank and Zoey have to figure out how to trick her in return.

Kathy Caple, it is abundantly clear, knows kids. She knows how they feel, how they act, and what motivates them. Her earlier picture books all address childlike concerns. In “The Biggest Nose,” an elephant is teased by her classmates because of the size of her trunk; in “Harry’s Smile,” Harry, the dog, decides to stop smiling because he doesn’t like the way his smile looks in photographs; in “The Purse,” a girl gives up her beloved Band-Aid box of coins because her older sister tells her it’s babyish. The desire not to appear babyish also plays a major part in the hippos’ revenge on their sister.

Caple’s talent lies in her straightforward approach to these concerns and in her enormously appealing watercolors of the characters that populate her stories. Hank and Zoey will win readers’ hearts with their earnest expressions--simply drawn but utterly unmistakable--of consternation, anger, surprise and sheer joy.

Sentenced to a morning of chores for landing a water balloon on their father’s newspaper (instead of on Dory’s head, where it was aimed), Hank and Zoey struggle gamely, their tongues hanging out in heat and disgust, through unenviable tasks: washing dishes, polishing silverware, sorting smelly socks (“a miserable job”). To add insult to injury, Dory is still lounging in their pool, contentedly spouting water, when they finish. So they put on their thinking caps, engineer Dory’s much-deserved comeuppance, and reclaim their watery oasis.

This is right out of real life. And “The Coolest Place in Town” may well be Caple’s best effort yet: It’s tightly plotted, filled with dramatic tension and completely hilarious. The pictures, with their warm green, blue and yellow tones, extend perfectly the humor of the story. At a time when booksellers’ shelves are laden with so many retellings of familiar classics, this story stands out for its engaging, original charm.