Twilight Sparkle, after finding out that she is immortal, decides that this is an excellent opportunity to learn about how the pony body works. What she didn't count on was Spike walking in during the process. And then things just get weird.

Contains: Extremely graphic yet non painful/lethal gore. And technically a bit of non-con but really it's that first one you need to watch out for

(This story is being reuploaded after a false plagiarism claim. To be clear: I am the original and sole author of this story, which was first written for a 4chan thread, then put on a pastebin, before I tidied it up and posted it here.)