Book 11 in the Magic Tree House series (1998)
A novel by Mary Pope Osborne
Jack and Annie are whisked off to the vast plains of Africa, where they must solve Morgan le Fay's third magical riddle. But that's only the beginning! Once the riddle is solved, they still have to get past a pride of lions, a humongous herd of rampaging wildebeests, and one very hungry Masai warrior.