Tree houses Magic 44 Time travel 36 Brothers and sisters 8 History 8 Animals 4 Dogs 4 Adventure and adventurers 2 Bears 2 Birds 2 Building 2 Castles 2 Compassion 2 Dakota Indians 2 Depressions 2 Dinosaurs 2 Earthquakes 2 Endangered species 2 Eruption, 79 2 Friendship 2 Frogs 2 Frontier and pioneer life 2 Gorilla 2 Human-animal communication 2 Indians of North America 2 Jungle animals 2 Knights and knighthood 2 Marine animals 2 Middle Ages 2 Monasteries 2