The analogy I use to teach theme and variations form, is the idea of a Halloween costume. I explain that we are used to seeing each child as they are usually dressed for school. Suppose they go home and come back in a really weird costume. They are the same person, but they have totally changed. If we REALLY know them, we can very likely still know it's them, although it can sometimes be much harder to tell.

OK, now go home, change costumes, and come back. It's the same story. Go through several more changes. Then, tie it to "theme and variations," the theme being the student in their normal attire, then all the variations being the many "Halloween costumes" we dress the student up in.

This worked well. The kids really understood the idea!