In this well-knit crossover debut for young audiences, a popular Norwegian author crafts an airy farce from elements both familiar and offbeat—from new friends with wildly disparate personalities afflicted by big but really stupid bullies (with a father to match) to an eccentric inventor and encounters with a giant anaconda in the sewers of Oslo. No sooner do shy Lisa and her brash, pint-sized new neighbor Nilly (William) bond than they also hook up with lonely Doctor Proctor—creator of a marvelous powder that produces massive, britches-busting bursts of (odorless) intestinal gas. Nesbø takes this promising MacGuffin in directions more comical than gross, having his two young protagonists use the powder in clever ways to foil hulking nemeses Truls and Trym, escape the aforementioned anaconda and ultimately even provide festive explosions for the grand Norwegian Independence Day celebration. Readers will have blasts of their own cheering on the sturdy protagonists. Lowery’s childlike line drawings are too sparse to have a noticeable effect on this rib-tickling tale. (Fiction. 10-12)