Author's note: A few points before I begin. First off, welcome! I had this idea when I was looking up the age of the characters in How I Met Your Mother and found out Ted, Marshall, and Lily were all born in 1978, Barney in 1976, and Robin in 1980. When I remembered the Weasley twins were also born in 1978, I realized this could actually work as a one-shot, and then I had so many ideas I decided to start posting a series of them, which is what you'll see here. However, to make it easier, all of the characters were born in 1978. This will help with a potential Barney/Robin in the future (as in, before they graduate) and I also have no idea how old Tracy is in HIMYM canon but she'll be in this too, hopefully, at one point or another. Also, I'm updating these as I think of them and write them, so it won't be daily or weekly.

If there are any triggers, I will mark them, but for the beginning a lot of this stuff is fluff. The only triggers I'm marking for now are inappropriate comments from Barney. I'm rating the fic teen for this, especially since it might go into darker stuff, but there won't be anything more inappropriate than just Barney's comments. I might go into some canon events from both HP and HIMYM, and they'll definitely be interacting with canon characters (you'll see that in this first one-shot) but for now, enjoy!

(also HIMYM and HP characters do not belong to me)

The double-prank

Five boys piled into the dorm room, looking around at the beds. The new first-year Gryffindor Boys were all exhausted, but not all of them were planning on sleeping just yet. Specifically, two red-headed identical twins and a boy with blonde hair.

Those three boys had been sitting with their older brothers, and had been sitting near each other, so they had met already. The boy with dark brown hair and the tall boy with lighter brown hair had already met and were friends. But the groups hadn't met just yet.

"Identical twins," said the dark-haired boy. "That's going to be interesting." He turned his head to one side, as if examining them for a difference.

"Well, everyone gets us mixed up," said one twin.

"Even our Mum does," added the other.

"Do you have birthmarks, though?" he asked. "That'll make it easier."

Both twins shrugged.

"Well, before we start unpacking, we should make sure everyone has the beds they want," Marshall, the tall boy, suggested.

Five beds, all four-poster, were arranged in a circle with trunks in front of them. The twins' beds were right next to each other, closest to the door. The next one in the circle, moving away from the door, was a trunk labeled "B. Stinson," then one labled "T. E. M.," and finally one labled, "M. Erikson."

"I'm good with this," said Barney, the blonde-haired boy. "But unpacking's boring. Why can't we explore anything?"

"Who says we can't?" asked one of the twins. This one had a pattern of freckles on his face that seemed easy enough to recognize. More freckles than the other twin, anyway.

"Charlie's got a shift tonight, don't forget that," said the other.

The first wrinkled his nose. "Really? That sucks. You know, by the time Charlie leaves school, Percy's already Prefect… he'll be way stricter…"

"Your older brother is a Prefect, or Head Boy?" asked Ted.

"Oldest was both, but he graduated last year," said the first twin. "Next is only Prefect, but he's sixth year— too young for Head Boy. And our other older brother, Percy, is too young for Prefect, but he's bound for it."

Both twins rolled their eyes. "He really is a pain sometimes, you know," said the second twin.

"By the way, I'm Fred," said the first one.

"And I'm George," said the second.

"Theodore Moseby, but I go by Ted," Ted said.

"Marshall Erikson," said Marshall. "I'm a muggle-born, so I don't know anything much…"

The twins grinned. "You two— what say you, along with Barney here— we show you how to live?"

Ted and Marshall glanced at each other. "What do you mean?" Marshall asked.

"Meaning we show you how to live life by having fun," Barney said. "You know, getting girls and playing pranks."

"See, George?" Fred asked. "Told you this was our guy."

"Well…" Ted trailed off in thought. "Okay, I'm in."

"Me too," Marshall said, though a bit uncertainty.

Fred clapped his hands together. "Alright! First order of business: sneak out and piss off Filch."

Marshall frowned. "Who's Filch?" Nobody listened.

"What? Now?" Ted asked. "We… we need to unpack!"

"Don't procrastinate," George said. "Besides, we need to explore a bit, don't we? Come on."

And with that, the twins led the way down to the common room past the portrait of the Fat Lady.

The five of them snuck down the stairs, and tried to find their way around. They had to duck out of sight every time a Prefect saw them, and once hid in a bathroom that they suddenly realized was a girls' room, though it was empty.

After maybe an hour of exploring, they found Filch trying to catch students out of bed near… they didn't even know where they were. How they were going to get back, none of them had any idea.

One of the twins—nobody could tell in the darkness— pressed a finger to his mouth, whispered, "Shhh…" and then took something out of his pocket and threw it at Filch.

The other twin gestured for them to run in the opposite direction, and they did— until Marshall, in the front, slammed into someone else.

Filch let out an extremely irritated yell, and the other person whispered, "Run!" and they kept running.

When they reached a corner, the other person told them to stop. The voice was female, and now that they'd stopped running, they got a good look at her— she seemed to be in her fifth or sixth year.

She poked her head around the corner and then looked back. "You guys wanna help me prank my friends?" she whispered.

One twin poked his head around the corner and when he looked back, his eyes widened, he said, "You mean my brother and whoever that is? Heck yeah."

The girl raised her eyebrows. "Wait, you're Charlie's brother? So one of the twins?"

"Both are here, actually," said the other twin.

"Alright, anyway," the girl continued, "lemme give you the rundown. A Gryffindor Prefect and a Hufflepuff Prefect— my friend Charlie and the Hufflepuff Prefect named Chiara. Chiara's a bit stricter than Charlie, but you don't want to be caught by either of them. Currently, they're patrolling the corridors right around this corner here. So?"

"We already pranked Filch," said Ted. "I'm not getting in any more trouble."

"Same here," Marshall whispered.

"You guys really are bad trainees," Barney said, rolling his eyes. "Come on, have some fun! And besides, you meet a girl in the corridors in the middle of the night, what're you going to do?"

"Excuse me?" the girl demanded. "I'm sixteen, for your information. And you're… what, a first-year?"

Barney looked like he was trying to say something clever, but was failing.

"That's what I thought," continued the girl. "Anyway, you in?"

"What're we going to do?" Fred and/or George said. "We've got extra dungbombs…"

"Won't be needed. I got Peeves' attention, and he's going to wreak havoc on them, but you guys need to help me out. One of you is a poor first year who needs to find the hospital wing, but I'll talk for you—"

"What do you mean? You can't pass for a first-year," Barney pointed out.

"Oh, but I can," said the girl mischievously. "You'll see. So after that, one of them'll lead you to the hospital wing. Then we'll run once we reach nearby it—I'll tell you when— and they're gonna be so mad, but they'll never know it's me unless I do something stupid tomorrow… honestly, I probably will. Anyway, sounds cool?"

"It should be the smallest of us," Marshall said, "so they look the most vulnerable."

"You're just saying that because you're the tallest," Barney said. "But you've got a point. Ted, get out there."

"Why me?" Ted asked, but they all ignored him.

The girl suddenly became shorter, and her hair was short and blonde— she'd taken Barney's form.

"Wait, you're that girl Charlie told us about? That can change her appearance?" One twin asked.

"Which is wicked cool, by the way," added the other twin.

The girl nodded. "I go by Tonks, not my first name, ever, so learn that," she said, though in her voice. Barney looked completely freaked out.

"I can try to do a deeper voice, but I don't have too much control over that, so

I'll do my best," Tonks continued. "Anyway. Ted, you have a bad stomach ache. Ready?"

"No," said Ted, but Tonks didn't listen, and dragged him out of the corner, and Ted was forced to pretend he had a bad stomach ache.

"Excuse me?" Tonks asked, but with a voice like a boy's. "Where's the hospital wing? My friend has a bad stomach ache."

"I can show you," said the boy, who must have been Charlie. He looked like his brothers with red hair, only many more freckles.

"Great, thanks," said Tonks, still with the deep voice.

"I'll stay here," said the girl, Chiara. "Stay patrolling."

Charlie led them around a few corners, and then eventually, Tonks whispered, "Run."

The two ran the other way, and Charlie turned around and then screamed as something came piling down on him. Peeves' laughing filled the corridor, and Ted glanced back to see both of them.

"I'll lead you back to the common room," Tonks said as soon as she met the others, "now get out of here before Chiara gets back from finding out what happened!"

That must have been why she'd disappeared, because Chiara wasn't there.

They ran with Tonks while they tried to find the entrance to the common room and finally found it, thanking her and running in.

"How did she know where the common room was?" Marshall asked.

"Beats me, but we stayed out of trouble," said Barney.

Ted looked shaken. "I… I think I'm just going to go to sleep now."

With that, he walked up the stairs and went into the dorm.

"Two people in one night!" said George when they got back up. "That's awesome."

"We need to set a record," Fred told him. "Like, high scores or something."

"We can make it a competition," Barney suggested.

"Or we can sleep," Ted said. He was already dressed for bed, and Marshall was too. "Figure it out tomorrow?"

"Fine," the twins and Barney groaned in unison. They got ready for bed and then went to sleep, even though they didn't want to.

And though Ted and Marshall disagreed, the next morning the twins and Barney suggested more pranking, and that was what they ended up doing.