The prompts for this were an ad for matching couples underwear I saw online, and a post that said "'That's my girl' might be the cutest thing a guy could say."

Warnings: Implied adult sexual relations.

"Annabeth, I got you a gift!"

She eyed her boyfriend suspiciously from her place on the couch, where she was working on a paper for one of her classes. "It's not my birthday."

"I know."

"Or our anniversary."

"I know."

"Then either you've done something stupid or are about to do something stupid," she decided.

"No, no! Can't I just get my girl a gift to make her smile?"

Although still suspicious of him, Annabeth couldn't help but smile at his words. She loved when he called her "his girl."Calling her "his girl" might just be the cutest thing he could call her, even if "Wise Girl" was her official nickname.

"I guess so. It's just that you haven't before. Which is why I am completely unconvinced that this doesn't come with some kind of caveat."

He imitated his girlfriend and rolled his eyes in the most Annabeth way possible. "Can you just pretend you trust me for thirty seconds and open it?" he teased her, plopping down on the couch next to her.

She took the box from him hesitantly. "Percy, if there are spiders in this box-"

"There aren't. You swore you'd send me back to Tartarus if I did that again, and I believe you." She gave him a pointed look that silently said I absolutely would send you back there if this box has spiders in it. "Just open it."

She sighed. He looked like a kid on Christmas morning. This could not be good.

She opened the box. Inside, was a yellow piece of cloth with owls splashed across it. She took the cloth out of the box.

"UNDERWEAR, Percy!?" You got me UNDERWEAR!?" she exclaimed. Out of all the things she was expecting, this wasn't one of them.

She held up the boy shorts in disbelief, and looked to her boyfriend for an explanation.

"Not just underwear for you!" He reached inside the box, and pulled out a matching pair of boxer briefs. "There's underwear for me too! We now have matching underwear!" he explained proudly.

"Percy, what in Poseidon's name would make you think that I wanted my underwear to match yours?"

"I just found them online and they were so cute, and I thought it would be fun if we had matching underwear!"

"So we're one of those couples now, huh?"

"What do you mean?"

"One of those couples who wear matching clothes. Gods, it's like we're already married."

Percy laughed. "I mean, I do plan on marrying you one day."

"You won't get the chance if you insist on making me wear these."

"Come on, Annabeth, be a good sport!"

"What on earth would make you think that I want to think of my mother while putting on underwear?"

He blinked. He hadn't thought of that.

"And—" she gave him a seductive smile. "Would I want to think of my mother if I'm trying to take these off of you?"

He blushed. He really hadn't thought of that.

She laughed at his discomfort. "Now do you understand why this makes for a ridiculous gift?"

He straightened up and tried to look somewhat intimidating. To Annabeth, he just looked like a petulant child. "I see your points, but I still maintain that this was a fantastic gift idea!"

"Keep telling yourself that, Seaweed Brain." She pulled the pillow out from behind her and smacked him with it. "Now get out of here and let me finish my paper."

He stood up with a sigh. "This isn't over, Wise Girl. You will wear the underwear."

"Absolutely not," she replied, already having turned her attention back to her essay. "But there is one thing you're right about."

"Oh?"

Her eyes glimmered mischievously. "This isn't over."

"Open it, Percy!"

"Yea, I bet she got you something awesome!"

Ah, yes. Things were going exactly according to plan.

It was Percy's 21st birthday, and all their friends had crammed into their tiny off-campus apartment to celebrate this milestone event. Everyone was laughing and enjoying each other's company (many of them laughing more than usual thanks to the available alcohol).

Annabeth had purposely made a show of presenting her boyfriend with his birthday gift. She wanted their friends to notice, and they played into to Annabeth's little game beautifully.

"Yea, Percy, why don't you open it?" she said sweetly, batting her eyes at him. "I'm sure everyone would love to see what I got you."

Without hesitation, Percy ripped into the wrapping paper. He excitedly lifted the lid on the box, only for his face to quickly fall once he saw what was inside.

"What is it Percy!?" Grover called.

"Yea, we wanna see!" Leo chimed in.

"Come on, Percy," Annabeth cooed, trying hard to keep a straight face. "Why don't you show our friends what I got you?"

Begrudgingly, Percy lifted the gift from the box, holding it between his index finger and thumb, as if it were something repulsive he didn't want to be touching.

A roar of laughter erupted from the crowd when they saw what he was holding.

Between his fingers, for all their friends to see, was a pair of bright blue boxer briefs, with clownfish all over them.

"Annabeth, how could you?" he whispered to her.

"I have no idea what you mean, babe," she said innocently. "I just thought it'd be cute if we had matching underwear!"

"Matching?" Someone-she thinks maybe Thalia-called out curiously.

Annabeth quickly snapped the box shut. "Yes! It's a matching set for me and Percy. But this isn't my birthday, so you only get to see Percy's part of the gift!"

"Aw, that's not fair!" Jason called out.

"Let it go, Grace." He shot his cousin a murderous glare. "Now that everyone's had a laugh at my expense, can we get back to the party?"

The group didn't need to be told twice. They all went back to their personal conversations. Now that their friends were distracted, Percy yanked Annabeth into the kitchen.

"Wow, you really got me, Chase," he told her, trying to look wounded but failing miserably.

"Do you like your birthday gift, Percy?" She teased him, snaking her hands around his neck.

"You really had to put my favorite fish on a pair of underwear? And make me show it to all of our friends?"

"It was only fair, after the lovely gift you gave me a few months ago." He rolled his eyes at her good-naturedly.

"So, where's the matching pair?" he questioned her. "Since we both know it wasn't in that box."

"Hm, I'm not sure," she replied, looking uncertain. "Maybe you'll find them when you're undressing me tonight."

Percy's jaw dropped, and then broke into a huge grin. "No way."

"Guess you'll just have to wait and see!" She laughed, and quickly slipped out of his arms and headed back towards their friends.

His girlfriend really was full of surprises.

"Annabeth, wait! Can't we kick them all out now?"

"No, you'll just have to wait. It's part of your punishment. Happy birthday, Percy."

She turned and left him standing in the kitchen, dumbfounded. This woman would be the death of him. But as he stood there, rooted to the spot where she left him, he only had one thought: Now they had two sets of matching underwear. And tonight, he gets to peel one off of her.

It was going to be a happy birthday, indeed.