"Knock knock."

Luna stopped in her tracks and turned to Harry, staring at him curiously.

Silence.

"It's your turn," urged Harry.

Luna tilted her head slightly. "My turn for what?" she asked airily.

Harry stared at her. "Well come on," he said incredulously, "don't tell me you've never heard of a 'knock-knock' joke!"

Luna shook her head. "I haven't had the pleasure," she said dreamily. "Am I supposed to knock you over the head with something?"

Harry laughed. "No, thing like that. We go back and forth, it's – well, you'll see. Just pretend I'm knocking at the door."

Luna shrugged and nodded. "All right, if you like."

Harry drew himself up straight. "Knock knock."

"Well hello, Harry," she said, a dreamy smile dawning on her features. "It's sweet of you to drop by. How are you getting on?"

Harry gaped at her.

"Aren't you supposed to answer now?" she asked.

"No – I mean yes, but," blurted Harry, "you don't know it's me."

"Of course I do," countered Luna serenely. "You're standing right there – "

"Okay, look," began Harry, "pretend you don't know it's me, all right? You're the one with all that imagination, just pretend there's a door – " he thought carefully, "– a closed door, and you don't know who's on the other side."

Luna stared at him blankly for a few moments. "I suppose that's easy enough," she aired.

"Right, we'll start over," said Harry, feeling strange at having to explain such a common joke to Luna. "Knock knock."

Luna motioned as though turning a knob before opening an imaginary door. "Well hello, Harry – "

"No!" exclaimed Harry, fighting a growing urge to giggle. "No, no, that's not how – okay, we're going to pretend some more, all right? You don't open the door just like that – "

"Keeping it closed would be rude though," said Luna. "I should be more welcoming, I think."

"– yeah, well just, um, pretend that you heard a bunch of Muggle convicts escaped from a nearby prison somewhere, so you have to know who it is before you open the door. All right?"

Luna frowned. "This is a very odd joke," she said.

"Just humour me," said Harry. "Right. All set?"

Luna nodded.

"Knock knock."

Luna moved closer and pressed both palms against the imaginary door, turned her head slightly to one side and peered at him with one eye.

Harry let go a sharp breath. "Okay," he said in exasperation, "what are you doing now?"

"I'm looking at you through the peephole," answered Luna, "to make sure you're not an escaped Muggle convict lurking about the porch."

Harry ran both hands through his already mussed hair. "No," he said, "that's – there's no peephole, Luna."

"We have one at home," she said, drawing back into her normal posture.

"Well yeah, maybe so," replied Harry, "but let's just pretend you're not at home, all right? You're visiting somewhere, let's say."

"You know, Harry," said Luna, "maybe you should fill me in on the backstory. I think it would go much better if – "

"No, no, there's no backstory," said Harry, "just – you'll get it right this time. Just go ahead and pretend you're visiting somewhere where there's no peephole."

Luna looked thoughtful. "Well, all right."

Right. Here we go, thought Harry. "Knock knock."

Luna gracefully turned on the spot so that her back was to him. "Excuse me, but there's someone at your door," she said loudly.

"Arrgh!" growled Harry in frustration. "No, there's no one else there. You're the only one. Deserted old house. No peephole."

Luna turned to face him once more.

"Got it?"

Luna nodded.

Give me strength –

"Knock knock."

Luna stood there and simply clasped her hands behind her back, staring at the "door" and uttering not a sound.

Harry gasped and threw his hands up. "What are you doing?"

"Well, if I'm trespassing I wouldn't want to give myself away, I should think," she said. "After all, if – "

"No, no, no," said Harry. "Okay, look. When I say 'knock knock', you just answer 'Who's there?', all right? That's all."

"Well, it seems unwise," said Luna, "but if it will make you happy – "

"It will," chirped Harry, "really happy."

Luna smiled. "All right, Harry. Just for you."

He took a deep breath. "Knock knock."

"Who's there?"

Yes!!!

Harry opened his mouth and gazed at Luna in preparation for his reply.

Luna tilted her head. "Should I shout it out?"

Harry slowly shook his head. "I can't believe this," he said in between giggles. "I can't remember how it goes!"

"Well, it's no wonder," said Luna, "you've changed the story so many times – "