Hey guys! Warning, there are spoilers to CAtU in this. Enjoy!

I know you don't know what I mean

But you're not going to know until you turn eighteen

It's called adulting!

- "Adulting" from Phineas and Ferb: Candace Against the Universe

Isabella could easily remember the first time she ever met Doofenshmirtz. They were on an alien planet together. How couldn't she remember?

The second time she had interacted with Doof was during the Pharmacist apocalypse. She hadn't really expected to see the scientist again, but there they were. And what made the encounter even weirder was that everyone was somehow turning into Doof.

The third time was brief. Some pistachio monsters had taken over the world and replaced everyone with look alikes. But they quickly put an end to that.

There was some distance between the third time and the fourth time.

A week after Isabella's sixteenth birthday. She stared wide eyed at what her mother put in her hand. She looked up from her hand to her mom, grinning. "Are you serious?" she asked, on the verge of squealing. Her mother nodded. Isabella took one more glance at the keys in her hand then threw her arms around her mom. "Thank you! Thank you! Thank you!" she shouted.

Isabella dashed out of the house as fast as she could. Her heart skipped a beat at the sight of the deep pink car in the driveway. She circled it, her mouth wide open. Her hands hovered over the car hood. "Oh. My. Goodness!" she let out. She opened the car door open and took a seat inside.

She took hold of the steering wheel and took in a breath. Her mother stepped outside and stood next to the car. Isabella rolled down the window. "Thank you so much mom!" she said, a smile across her face.

Her mother smiled back. "Want to take it for a ride?" she said.

Isabella gasped. She nodded quickly and slipped the key into the ignition.

Taking her car, she drove around her neighborhood, just to get a feel for it. She went past Phineas and Ferb's house. She looked at it, deep in memory. She hadn't been going over there as often as she used to. She just couldn't find time with getting a job at the restaurant and the driving tests. And she was sure he was busy too, so it was probably for the best. She let out a sigh and went on driving.

"Wait! Buford! That's important!" she heard Phineas say from the backyard. A screw went flying through the air from their yard, rolling into the road. It was too quick for Isabella to stop in time, and the screw punctured her front tire.

She swerved a moment before realizing what had happened. She pulled over to the curb and got out of the car. Stepping out to the side, she clicked her tongue. There was a soft hissing as the air leaked out of the tire. "Dang it," she said under her breath. She put her hands on her hips and looked around for a solution.

A white, purple, and green truck pulled up behind her car. Printed on the side of the truck was the label D. . Isabella turned her head curiously. Out of the passenger side stepped Vanessa. She blew her bangs out of her face and stepped up to the Flynn-Fletcher residence. "Bye honey! Be safe!" someone called from the truck.

Isabella stepped up to the driver's window. "Doofenshmirtz? What are you doing here?" she asked, seeing it was him.

"Oh, I'm just dropping off Vanessa for her date," Doof said.

Isabella stopped a moment. She glanced back at the house. "Date?"

Doof nodded. "Yeah, with that Ferb kid," he said.

Isabella let out a sigh of relief. She had hoped it hadn't been with Phineas. Doof pulled her out of her thoughts. "So, whatcha doing?" he said.

Isabella looked back to her car and frowned. "I was trying out my new car, but I got a flat," she said sagging.

"You've got a car? What crazy person would give a sixteen year old a car!" Doof said.

Isabella raised an eyebrow. "Um... Says the person that drove their daughter to their date. Doesn't she have a car?" she said.

Doof rubbed the back of his neck. "Well, she doesn't have a car yet, but I'm planning on giving her one on her birthday. A real nice yellow one. Don't tell her," he said with a chuckle.

She laughed in response. "Wait, hang on, you said you've got a flat? I think I've got something that can help you with that," he said, getting out of his truck. He walked up to the front of Isabella's car, her following. He knelt down to get a better look at the tire.

He pulled a small gun shaped thing from his coat, making Isabella jump back warily. "Behold! My portable Re-Tire-Inator!" he explained.

Isabella loosened up a bit. She tilted her head curiously. "Retire? Like retirement?" she asked.

Doof shook his head and face palmed. "No! Why does everyone keep thinking that! The Re-Tire-inator can fix your flat," he said. He aimed the device at the car, causing Isabella to get nervous. If he messed up her brand new car, she didn't know what she would do. He took the shot, shooting a bright green laser.

Once it was over, Isabella checked the tire. "Wow! It really worked!" she said, hitting the full tire with her palm.

Doof put his hands on his hips. "See? Did I tell you or did I tell you?" he said proudly.

She got up and started to get back in her car. "Thanks Doof!" she said.

He smiled at her through the window. "Any time," he said.

And so the next two years went by, and Isabella ended up seeing Doof more and more. He became less of an acquaintance and more of a family friend. He was always there to help her and her family. She saw him about once every two months, most of the time just by coincidence.

It was the day before Isabella was off to college. She was eighteen, working a job at the restaurant and holding other programs in town. She had a bright future ahead of her, and though her and Phineas had grown farther and farther apart, Isabella was sure it just hadn't been meant to be. Her summers with the boy next door seemed to be just a memory of years ago.

Her mother stepped into her room. "Isa, dear, can you believe it? Tomorrow you'll be off to college," she said.

Isabella folded a few more shirts and stuffed them into her suitcase. She couldn't believe it either. "Mom, I love you so much. Thank you for always being there for me," she said, continuing to pack.

Her mom smiled and walked over to her. She took a t shirt from Isabella. "Isa, why don't I finish packing for you? You can go say goodbye to all your friends," Mrs. Garcia Shapiro offered.

Isabella grinned. "Thanks mom!"

"Make sure to make a list so you don't forget anybody. The college is a while away," her mother said.

Isabella nodded and grabbed a scrap piece of paper from a desk. She took a seat at her bed, next to her suitcase. "Let's see..." she thought aloud. "There's Holly, Gretchen, Millie, Katie, Buford, Baljeet, Ferb, Doof, Adyson..." Isabella tapped her pen to her chin thinking.

Mrs. Garcia Shapiro smirked and raised an eyebrow. "Forgetting anyone?" she said.

Isabella's face lit up. "Oh! Ginger! Can't believe I forgot her," she said.

Mrs. Garcia Shapiro clicked her tongue. "Anyone else?" Isabella smiled sadly, knowing who she was implying. "Isa, when are you going to talk to him?" her mother said.

"When I'm ready," Isabella said.

"If you keep waiting till you're ready, you're never going to see him again. Tomorrow you leave. This is your last chance," her mother said.

Isabella went around town, spending time with her friends. Her heat raced when she finally came to say goodbye to Ferb. She didn't know why, but she just couldn't get the guts to talk to Phineas again. It had been so long and she knew deep down she had been avoiding.

But for now, she would just say bye to Ferb. And if Phineas happened to be there too, she would wave or something.

Linda let her in and told her Ferb was in her room. She stepped up the stairs and knocked on the door. It was quickly opened by the green haired boy, a blank expression on his face.

"Hey Ferb," she said with a small wave. He gave a nod to acknowledge her. He allowed her into his room and she took a seat on Phineas' bed.

She finally decided to ask the burning question. "So... where's Phineas?" she said.

"I think he went to go pick up some parts for this machine he wants to build. He hasn't been able to pick a college yet, so he's building a machine to randomly pick for him," Ferb explained.

Isabella let out a breath of relief. That meant she wouldn't have to face him. Her eyes wandered to Phineas' desk. There was a large pile of college acceptance letters. No wonder he couldn't pick a college with so many options. Lots of colleges were after the inventive boy.

"I- I just wanted to stop by and say goodbye before I leave," she said, folding her hands in her lap nervously.

"Right, tomorrow you're off to college," Ferb said. Isabella nodded. There was a moment of awkward silence. Then Ferb sighed. "He still talks about you, you know," he said, addressing the elephant in the room.

Isabella looked down. "I'm just-"

"Scared," Ferb finished for her. She nodded. "You know, he would really appreciate it if you at least said goodbye," he added.

Isabella bit her lip. "I owe it to him," she said. She got up and headed towards the door.

Ferb stood up, following her out. "I always hoped you guys would get together," he said.

Outside the room, she turned back to face him. "Me too. But... life has a funny way of messing stuff up, huh?" she said, forcing a smile.

Ferb nodded. "Maybe. I'm sorry to see you off, Isabella. But hopefully we'll see each other again," he said. Isabella smiled warmly. There was something about the smoothness of his voice that made her believe for a split second that things would work out. But then she remembered that the chance of that happening was unlikely.

Isabella's next stop was Doof's. She rang the doorbell and waited. Her conversation with Ferb still rang in her mind. She was pulled out of her mind by the door opening.

"Isabella, what an unexpected surprise!" he said. "And by unexpected, I really do mean unexpected," he added. Isabella giggled and followed him inside.

"So, what brings you here?" he said, showing her to the kitchen. They sat across from each other at the kitchen table.

"Well, tomorrow I'm off to college. So I came to say goodbye," she said.

"That's so exciting!" Doof said, smiling.

"Yeah," Isabella said distantly. She traced a circle on the table boredly.

Doof raised an eyebrow. "You don't sound excited," he said.

Isabella looked up. She put on a smile. "I am super excited! It's college! My whole life ahead of me!"

"But?"

Isabella sighed. "But, I don't think I'm ready to let go just yet," she said.

"But you can call or text anytime," Doof said, a bit confused.

"I know, but... even if I do come back after college, it won't be the same. It'll have been years since I've seen him, and we'll be two completely different people!" she let out, throwing her hands out in front of her.

Doof smirked. "Him?" he said.

Isabella rolled her eyes at her mistake. "Phineas. There. I said it," she said.

"Whatever happened to you two? He used to be all you could talk about," Doof asked.

"Things... changed."

"Tell me what happened," Doof said. He leaned his head in his hands.

Isabella sighed. "He became more invested in his inventions, I became more invested in my job. We each had different things going on," she said, avoiding eye contact.

Doof rolled his eyes. "Yeah, like I'm going to believe that for one second," he said.

Isabella was taken back. "What? What do you mean?"

"You were scared. You held in those feelings for too long until you accepted that it would never happen," he said.

Her eyes widened in realisation. "You're right," she said sadly. She set her head down on the table. "But it doesn't matter anyways. If he wanted me, then he would've said something. So I guess it just wasn't meant to be."

"Did you ever think he was the one waiting on you?" Doof said.

Isabella went quiet. Then finally, she said, "I'm not ready."

"If you wait until you're ready, it'll never happen," he said.

Isabella sat up. "You're right."

"Adulting can be hard, but it's worth it," Doof added.

Isabella smirked. "It's still not a verb," she said.

Doof rolled his eyes. "Whatever. Let me let you in on a little secret," he said. "I'm still figuring out how adulting works," he said.

She giggled. "Yeah, I guess I've got a while to figure out adulting too," she said.

"Yes! She said it!" Doof grinned. He high fived himself.

Isabella got up from her seat. "I've got a few other people on my list, so I've got to go. I'll miss you," she said. They hugged before she left.

Isabella woke up bright and early. Today, after work, she would be going to college. After getting ready for the day, she stepped towards the door to leave. She slipped her hand into her pocket. She heard a crumple and she took out the page. It was the paper with the list of people to say goodbye to. She smiled sadly at the list. She took out her pen, realizing it was incomplete. She scribbled "Phineas" at the bottom. If she had time, she would talk to him. She had to adult up and do it.

She put the page back into her pocket and took in a breath. She reached for the door handle, but a letter slipped through the mail slot. She eyed it curiously and picked it up.

Opening the envelope, she found a note and a small gift wrapped shape. She read the note first.

To Isabella,

May all your future adventures be just as amazing as you are. Don't worry, you've got this. You could adult better than I ever could by age ten.

From H. Doofenshmirtz

Isabella smiled at the note and unwrapped the shape. It resembled a fireside patch. On it was an embroidered picture of Doofenshmirtz. "What is this?" she asked, examining the patch. She looked back at the note.

P.s.

You deserved an adulting patch, so I made one. Of course, I can't embroider, so my robot Norm did. You've earned it!

Isabella smiled and squeezed the patch tight in her hand. Then she stuck it in her purse. Adulting might not be easy, but if anyone could do it, it was Isabella.

Hope you guys like this! I fell in love with the Adulting song the moment I heard it and came up with this. It actually had more Phinabella than I had anticipated and it was just supposed to be Isabella and Doof talking. But I like how it turned out. Thanks for reading and thanks for the reviews, follows, and favorites!