“What do you get if you cross a pig and a centipede? Bacon and legs.” “Who did Dracula marry? The girl necks door.”    A parent may need to read the riddle, which lays out which hidden-in-plain sight objects to find. But there are so many items in the intricate photo illustrations that children can make up new riddles or enjoy a little freelance sleuthing. Children of all ages love these books. Both series have books geared specifically to preschoolers, though children in that age range may also enjoy the regular series.