Fred and George smelled a bit off to Harry, ever since he met them.

Harry had a very keen nose. One of his senses had to be the best when living with the Dursleys. It couldn't be sight, because he needed glasses. It couldn't be hearing, because of those untreated ear infections. Touch and taste weren't good for long distances. So Harry learned to train his nose—a fine skill for avoiding Dudley.

Anyway, Fred and George smelled weird, and Harry was going to find out why. First off, the scent wasn't entirely unique. He smelled it from the fifth, sixth, and seventh years, though no other third year. Harry's focus soon drifted from Fred and George, what with Quidditch, but smelling it on Lockhart in his second year renewed his interest.

What do Lockhart, fifth years, sixth years, seventh years, and Fred and George have in common? The answer came when Harry was wandering about one night and stumbled upon a moaning and grunting broom cupboard.

Okay, so they have...shagging...in common. The scent was of people who had recently shagged. The implication was a bit disconcerting since Harry smelled it on them all the time, but he tried to put it out of his mind. The new question was, who were Fred and George shagging? Couldn't be a fifth year—he hadn't smelled any fourth year with the scent last year. But the Chamber of Secrets got opened and that distracted Harry for the year.

He picked it back up the next year at the Welcoming Feast. Now that Fred and George were fifth years, it was stronger than ever. How did nobody else notice it? Maybe because they gradually introduced it? But Merlin, it was so strong.

Anyways, they had to be shagging seventh years. Maybe even the same seventh year? No, that was a bit weird. Harry operated on the assumption that there were two.

Black's escape wasn't enough to distract Harry this time. It'd been years since he started the investigation! Nothing could dissuade him now, and if it did, he just had to be around Fred and George from how strong the scent of shagging was. Harry went around the castle and sniffed every Gryffindor seventh year, then Ravenclaw and Hufflepuff, then Slytherin. Nothing. For good measure, he did the sixth years, too, though he doubted it was one of them.

Stuck and frustrated, Harry gave up because another problem showed up to bother him—Hogsmeade weekend. He couldn't go! Harry seethed but Fred and George piqued his interest when they dragged him to an unused part of the castle. Would he finally find out?

No, the twins were just giving him a map to get into Hogsmeade—the Marauder's Map. That was nice, but now Harry figured he could also finally find out who Fred and George were shagging.

The twins were sneaky little buggers and evaded him, but finally Harry caught them shagging...each other? Merlin, his face turned red. Of course. Late at night, he said the words to close the map quickly, then threw it on his bed and covered his face with his hands. Of course.

He tried to put it out of his mind, and it worked, for a while. But now that he knew they were shagging each other, and got the reminder from their scent, it was hard to be around them. Several weeks later, he walked down to the common room, using the map to be sure he left at the same time as the twins. He came up behind them and dragged them into an unused part of the castle.

"Fred and George," he started. "Shagging?" They turned bright red and shook their heads, but Harry put a hand up to stop it. "Too late, I already know. I'd forget about it, but it's just always there, whenever I'm around you, the smell. Could you...put on some cologne? Take a shower more often?"

They nodded.

"And, uh—" Harry sniffed for emphasis—"eat some more fruits and vegetables. Especially fruits. You smell...unhealthy." There. He embarrassed them, and then he made up for it by discreetly helping them. They'd see the difference sooner or later...or should he say, taste the difference.