"Mr. Twycross, sir?" Lily Moon asked, with her hand up. "Why do we turn on the spot to Apparate?"
"That's part of the Apparition process," the Ministry instructor told her – and, by extension, the whole class. "Part of Deliberation."
"That's not what I mean, though," Lily said. "That's just saying that we do it. I'm asking why we do it, because surely we wouldn't just do it for no reason?"
Mr. Twycross looked sort of annoyed, then frowned, and around the room there were a series of sighs as people stopped ferociously concentrating on their Destinations.
"It is because Apparition is, magically speaking, moving without moving," Mr. Twycross said eventually. "And to turn on the spot involves moving, but you do not actually change where you are – which is quite similar, though not exactly the same."
"Right, I think I get it," Justin decided, out loud, which only made Mr. Twycross look testier. "It's like how the asphodel flower is associated with the Greek afterlife, and wormwood is medicinal and poisonous, and sloth's brains are slow, and all those things are involved in the Draught of Living Death which seems like death but isn't."
Harry overheard Professor Snape, very quietly, give three points to Hufflepuff.