Turtles as a rule do not use their tails for the purposes mentioned for other reptiles. However, having a short, blunt tail, rather than no tail at all, can be vital for a turtle. Adult male turtles generally have longer, stouter tails than females and use their tails to hold onto the female's tail during the mating process. Identifying uses of reptile tails provides insight into some of the ecological purposes to which an appendage can be put. And indeed, although he may not have realized it, this is what the second-grader wanted to know.
Ecology is often a matter of simple questions and complex answers.
Not as whimsical as Kipling's explanations, but every bit as fascinating.