After a child begs for a pet, the child's mother finally says yes. But which animal will be the best pet? Using animal classification and habitat needs, the child narrows it down from Kingdom Animalia, from invertebrates to vertebrates. Reptiles and amphibians are out, and birds and fish are soon off the list. That leaves mammals, but which one? An elephant won't fit through the door, and a tiger would be too hard to walk. What's a child to do?