Deep within the grandeur of Wayne Manor, young Bruce Wayne sat on a plush sofa, engrossed in a comic book. His brow furrowed in concentration as he absorbed the vibrant illustrations.

Suddenly, the silence was broken by the impish laughter of Selina Kyle. She had an uncanny knack for mischief, and her sights were set on Bruce.

"Hey, Brucey!" Selina called out, a teasing lilt in her voice. Bruce immediately frowned, disliking the nickname she used.

Selina pranced over, a wicked grin plastered on her face. "What'cha reading, Brucey?" she asked, snatching away the comic book.

As Bruce reached out to retrieve his comic, Selina delivered a swift wedgie. Bruce squealed in surprise and discomfort, not understanding what just happened.

"Wh-what was that?" Bruce stammered, too startled and embarrassed to be angry. Selina laughed even harder at his confused expression.

"That, Brucey, was a wedgie! Haven't you ever got one before?" Selina asked, still giggling. Bruce shook his head, his cheeks turning crimson.

Selina made fun of Bruce's tighty whities. "Seriously, Brucey, those are so dorky!" she commented, doubling over with laughter.

Bruce, ever the innocent, didn't understand why his underwear was a matter of amusement. "But... they're just underwear," he defended, genuinely puzzled.

"Only babies and dorks wear tighty whities, Brucey. They look like a diaper," Selina explained, her laughter subsiding into a smirk.

Bruce frowned, examining his khakis as if he could see his underwear through the fabric. "I'm not a baby," he said, more to himself than to Selina.

Selina's smirk turned into a devilish grin as she ruffled his hair. "Prove it, Brucey," she said. "Stop wearing those dorky tighty whities."

"But... but... I'm a big boy!" Bruce protested, squirming under Selina's gaze. His voice wavered, his innocence shining through his defiance.

"Big boys don't wear diapers, Brucey," Selina retorted, playfully poking his chest. "Big boys wear boxers."

Bruce, naive as he was, did not understand what boxers were. Selina sighed, "They're a type of underwear, silly!"

Selina decided to double down on her teasing, treating Bruce like a baby. Bruce protested, "I'm not a baby, Selina!"

Selina laughed, "Oh, Brucey, you're such a crybaby!" Her laughter echoed through the room, making Bruce more embarrassed.

Bruce looked down, his cheeks flushing. "I'm not a dork... I'm not a crybaby," he mumbled. His innocence was slowly giving way to determination.

Selina's laughter died down as she noticed Bruce's growing determination. "Well, Brucey, we'll see," she said, patting his flushed face.

"I'll prove it to you, Selina," Bruce said. He looked up, meeting Selina's eyes for the first time. "I'm not a dork, I'm not a baby, and I'm not a crybaby."

Selina watched him in surprise. She saw a spark in his eyes she had never seen before. "Well, then. Let's see, Brucey," she said, playfully challenging him.

Bruce stood tall, a newfound resolution in his heart. "I'll show you, Selina," he said firmly. His weak, timid nature seemed to vanish.

Selina gave him a nod of approval. "Bring it on, Brucey," she said, her mischievous smirk making a return.

With a newfound courage, Bruce was ready to face any challenge that Selina might throw his way. After all, he was a big boy now.

As the day ended, Selina left the manor with a promise of return. Bruce watched her leave, resolute in his commitment to prove himself.

From that day forward, Bruce decided to leave his childish tighty whities behind. After all, he was determined to prove he's a big boy to Selina.

Bruce's actions weren't unnoticed. Selina returned to the manor, finding a different Bruce; one who was ready to stand up for himself.

Selina saw the change in Bruce. And even though they continued their playful banter, she found herself silently approving of this new Bruce.

The adventures of Bruce and Selina continued with new respect for each other. Their friendship, though filled with teases, grew stronger.

In spite of all the mockery, Bruce learned an important lesson from Selina. He learned to stand up for himself, to be stronger, and most importantly, to be a better friend.

So, in his own way, Bruce became a big boy. It was a journey full of laughter and lessons, one that shaped him to be who he was meant to be.

And though Selina continued to tease him, Bruce took it in stride. He wasn't a dork, or a crybaby, or a little boy anymore. Bruce Wayne had grown up.

Yet, the most significant change was in their friendship. Bruce's innocence and Selina's mischief, though poles apart, created a bond that was bound to last a lifetime.

So, in the grandeur of Wayne Manor, the adventures of Bruce and Selina continued, their friendship strengthening with each passing day. And it all started with a simple wedgie.

In the end, Bruce realized that his friendship with Selina, despite the teasing, was the most valuable lesson of all: to stand up for himself, to face his fears, and to grow up.