Have you ever seen that show Unlikely Animal Friendships? Dogs will be friends with lions and bears will be friends with giraffes. It's pretty fascinating, precisely because it is so unnatural.
I've always enjoyed stories with unlikely friendships, but I hadn't played around much with writing my own until pretty recently while writing the sequel to The Closing Prophecy. I realized that these relationships are not only way more interesting than the expected friendships, but they're way more fun to write. There's tension when you have two characters who are from different worlds--whatever that may look like--and it's rewarding to see their connections unfold.
This prompt may require you to already have a story or a character in mind. Give the character a friend who she should NOT be friends with in the natural world. You may envision this as a chapter or a background piece, or the relationship may become central to your story, but I promise, it will give you good stuff!
As always, feel free to share how you tackled this and how it went!