With the help of four volunteers, learn about the anatomy of changelings. Not the interesting stuff, like eyes or ears. Nah, just the boring stuff, their reproductive organs. Which make up two of the volunteers. The other two are normal ponies who will interact with the changelings to demonstrate how everything works.

This is a one-off set in the style of a school, only, not really. Don't tell the volunteers the "professor" isn't really a professor.