The Wicked Witch of the West is killed when water is poured on her head.
I don't remember any other fairy tales where this happens. Did Baum invent this method for killing witches?
Richard provided/found the clip/quote:
". . . the Witch began to shrink and fall away. "See what you have done!" she screamed. "In a minute I shall melt away."
"I am very sorry, indeed," said Dorothy, who was truly frightened to see the Witch actually melting away like brown sugar before her eyes.
"Didn't you know water would be the end of me?" asked the Witch, in a wailing, despairing voice.
"Of course not," answered Dorothy; "how should I?""
Apparently Dorothy didn't know about it, but did the world know about it?