``What do you call a stubborn emu? An emule!''; ``...a rhinoceros that hides its head in the sand? A rhinocerostrich!'' Once they get the idea, young riddle-fanciers will enjoy being able to guess many of the 15 answers. Most's boldly drawn, cartoon-style illustrations are lively and amusing; he's content to show both animals in the same activity: no weird mixed breeds! Not essential, but the wordplay here is entertaining. (Picture book. 5-8)