An eye- (not to mention mouth-) opening look into a common yet mysterious activity."No, bugs don't yawn," as Kim pithily puts it, but nearly all other large animals, including penguins, snakes, and guinea pigs, do. Humans begin just 12 to 14 weeks after conception, more often as we grow and then less as we age. Why? There are no clear answers, but, as she observes, people often do it when bored, fatigued, hungry, or focusing on a task. Also, and perhaps most mysteriously of all, it's mightily contagious, as anyone viewing the author's neatly drawn, geometrically exact scenes of daintily yawning, diverse figures will discover. Picture books from Susan Bonners' Just in Passing (1989) on have conveyed the same lesson but usually as a means of pitching a wakeful tyke willy-nilly into slumberland. Here, past victims or caregivers curious about how that happens will get the same effect but with some background facts and findings on the phenomenon added in and capped with an unusually large list of sources cited. (This book was reviewed digitally.)Informative and, intrinsically, interactive. (Informational picture book. 4-7)