【明報專訊】"If vampires don't exist, how do you account for（解釋）the one Uncle Felix caught in our garden?" Teddy asked. "Can you nail down your description of that vampire a little more exactly? I mean, tell me what it does and what it looks like?" "Oh, it's just a typical vampire, the kind you see in the movies," Teddy answered. Nail down here refers to giving a clear and precise description（描述）of someone, a situation, a problem, etc.