In the novel by L. Frank Baum, the Good Witch of the North does not know how Dorothy can get back to Kansas, so she uses her magic to conjure a chalkboard with instructions for Dorothy to go to the Emerald City.
The following quotation highlights this:
As for the little old woman, she took off her cap and balanced the point on the end of her nose, while she counted “One, two, three” in a solemn voice. At once the cap changed to a slate, on which was written in big, white chalk marks:
“LET DOROTHY GO TO THE CITY OF EMERALDS”
The little old woman took the slate from her nose, and having read the words on it, asked, “Is your name Dorothy, my dear?”
“Yes,” answered the child, looking up and drying her tears.
“Then you must go to the City of Emeralds. Perhaps Oz will help you.”
My question is: why does the chalkboard instruct Dorothy to go to the Emerald City when the silver shoes are capable of sending her home? Is this chalkboard all-knowing? If so, why send Dorothy on a wild goose chase? And if it is not all-knowing, where do you suppose it got this advice from?