"No questions?" Risu asked, looking around. "Alright. Great! You shouldn't think about this too deeply, actually. You'll just overcomplicate things for yourself. From our perspective, the apparent paradox of the Earth moving and standing still at the same time is a rather, well, commonplace, maybe even boring observation. Einsteinian relativity is a little more complex than that, I'm afraid, but we've made our first step."

He paused briefly, structuring his following comments. "Perhaps I should talk a little bit about Galileo's insight. As I said, he realised that, no matter how you do it, there is no absolute way to determine whether something is standing still or moving. It's only relative to something else that we can talk about stillness or motion. In other words, he got rid of the idea of absolute stillness." He shrugged. "Or, of course, motion. We've come across the term 'absolute' a little while ago already, and Einstein removed the absolute from time and space.

"But, to stay with Galileo for one more moment. Let's see if we've all understood this concept. Relative motion and stillness. Let's say you're on the Hogwarts Express, with a book on the table in front of you. The book would seem to be fairly motionless, correct? But!" He raised a finger. "What about a person waiting at a crossing? When the Express thunders past, is the book still motionless? Or..." He paused, and smiled brightly. "Or does it depend on the perspective? Can we say that the book is definitely still, or definitely moving?"