Final answer:
The clouds were fashioned from Ymir's brains, according to Norse mythology as told in the Prose Edda.
Step-by-step explanation:
The question "What came from Ymir's brains?" refers to a story from Norse mythology found within the Prose Edda, a collection of Old Norse poems and stories about the Norse gods and their creation myths. According to the Edda, after the giant Ymir was killed by the gods Odin, Vili, and Ve, they fashioned the world from his remains. From Ymir's brains, they created the clouds. This mythological tale symbolizes the link between human intellect and the formless and shifting nature of the clouds, representative of thoughts and ideas.