Since Harry had lived in Hogwarts for the years that Papa had spent teaching, he was well affiliated with the castle and all its tricks and secrets, including the House Elves. They were obsessed with the 'famous master Harry Potter', and he wasn't one to pass by an opportunity like that. He dragged Neville and Draco with him the next time he visited the kitchen.
"What are we doing here?" Draco hissed. Harry grinned, and tickled the pear. It turned into a doorknob, and Neville gasped. He opened the door.
"Welcome to the Young Marauder's Base." He announced grandly. The House Elves squeaked with delight as he entered, and they were swarmed. They brought over hot chocolate, which he asked for without fail every time he visited, and they settled around a table in the corner. He took a sip, and sighed happily.
"This is what we're going to do." He planted a potion on the table.
"It's a beard growing potion. We're going to put it into tonight's pumpkin juice. The beards fall off after a few minutes."
"That's fantastic." Draco acknowledged with a grin. Neville giggled, but glanced around.
"Won't they notice?" Harry laughed.
"Yeah. But they know me. So I might be able to persuade them not to tell."
"Mimsy!" The Elf skipped over.
"How is Mimsy to be helping young master Harry?"
"Say I were to put a temporary beard growing potion in the pumpkin juice for this evening… Would anybody find out about it?" Mimsy glanced at the jar and tugged her ears. Harry splashed a drop into his hot chocolate and took a swig. He instantly grew a long white beard that rivalled Dumbledore's. She giggled.
"That would be a very funny prank Master Harry." A couple of the other elves took notice and joined her giggling. After a while the beard faded away.
"It's perfectly safe." He looked around the room.
"Will you keep our secret?" One elf bounced up and down on the spot.
"Blissy is finding the beard very funny. Blissy is wanting a beard of her own. Blissy will keep your secret." The other elves nodded. Harry passed them the hot chocolate, which they each sipped from and grew hilariously long beards that brushed the floor. He shared a grin with Neville and Draco. He skipped over to the pumpkin juice, and all the elves turned away so that they could safely say that they hadn't seen anyone put anything in it. They were clever like that. He poured it in.
"We were never here." Draco said, and they ran out the door and down the hall.
"This is going to be hilarious." Neville squeaked.
"I'll tell Crabbe and Goyle that I overheard the twins planning a prank, and to keep it secret." Harry frowned.
"If I tell them to keep it a secret they won't be able to resist telling Theo, and Theo will tell Pansy, and Pansy is the biggest gossip in the school." He explained. Harry nodded.
"Sneaky."
Harry was sat, eating dinner, avoiding the pumpkin juice, when word reached the Gryffindor table that the Weasley Twins had a prank planned. The twins looked at each other in confusion, as if to say; did we? Harry smirked. Then the screaming started.
"Get it off, get it off!" One girl shrieked. Harry winked at Draco, and tried to keep a straight face, before glancing around. At the teacher's table Papa stared at his beard in horror as students sniggered, and charmed it off. Dumbledore peered around, a twinkle in his eye as he inspected each person's beard, and his own, which had grown even longer. Hooch twirled hers happily, and Flitwick was attempting to make a scarf out of his. All around students were growing beards, and attempting get rid of them. Pansy Parkinson was crying as she inspected herself in a pocket mirror. Harry could see the twins wincing under a fourth year girl's wrath as she gestured to her beard and screamed at them. They were holding their hands up, proclaiming their innocence. But really, the whole school knew: who else would have the nerve to pull such a prank? Certainly not innocent little Harry Potter.
Eventually Dumbledore stood up.
"Attention. It appears we have been the victims of a minor comedic prank. Never fear, the effects will wear off momentarily. Please calm yourselves." Even as he spoke the beards were fading.
"Marauder's one, Weasley's none." Harry whispered to Draco. They glanced over to Neville, who threw them a discreet thumbs up. Mission accomplished.