Eren walked into the library to discover two very important things.

One, Armin was there.

Two, no one else was.

One plus two equaled a great place to have sex, in Eren's very expert opinion as a professional horny sixteen-year-old. Armin seemed less convinced of Eren's arithmetic when he sidled up behind him to grab his ass.

"Eren!" he yelped. "We're in the—"

"Library. Yeah." Eren's hands snuck around to Armin's front, to smooth down his hip and to his thighs. "No one else is, though."

Armin gave another murmur of protest, this one not as firm as the last, especially when Eren's lips came down to brush against the shell of his ear. He grumbled and leaned a bit heavier against the bookshelf when Eren tilted his hips forward to rub on him.

"You're ridiculous." Armin gave a little ah as Eren's lips and teeth found his neck. "Did you…did you at least lock the door?"

Eren's hands wandered up the insides of Armin's thighs, groped upward to rub between his legs. He let out a groan as the motion encouraged Armin to rub backward against his dick. Fuck, was Armin's ass amazing, and talented in so many different ways.

"I…think I shut it—"

"You didn't," came Mikasa's flat voice.

Armin stiffened and jumped as if he'd been scalded, and cracked the back of his head against Eren's nose. Eren reeled backward into the facing bookshelf. He growled and pressed his hand to his aching face, glowering at Mikasa through the tears of pain and thwarted dick-on-ass action.

"You didn't have to stop," Mikasa said calmly. "I was just informing you."

"Yeah, thanks a bunch." Eren winced and wiped the blood from his nose off on his hand. Armin made a small, concerned noise, and approached with a handkerchief from his pocket.

"Should I carry you to the infirmary now or wait until you've completed copulating?"

Eren swore at the stab of pain from Armin dabbing at his nose, as well as Mikasa's flagrant disrespect for his ambitions of dry-humping Armin against a bookshelf. Armin gave her a small, red-faced smile from underneath his bangs.

"I—I think the moment has passed," he explained.

Mikasa's head tilted to the side. "Could I be of any assistance? I could bring you a snack from the mess hall to set the mood."

"FUCKING LEAVE US ALONE," Eren half-screamed, half-cried.

Mikasa did not, and hauled a struggling, swearing Eren to the infirmary by his ankles.