A/N: Writing this for funsies, and also because little Don is just so cute and the B Team pair is my ultimate weakness. :p It'll probably be a few chapters long!

Mikey let out an oof when Donnie abruptly jumped into his lap, one arm coming up automatically to catch the much smaller turtle before he could fall. The kid was in full Worm Mode, squirming and resituating until he had himself tucked neatly under Mikey's chin, both his little arms wrapped tight around something he was hugging protectively to his chest.

Good thing Mikey had a shell, or he'd be more black and blue than he was green by the time this whole turtlesitting thing was over.

Off somewhere in the distance—he could hear it plain as day, now that he was listening—Raph and Leo were arguing in their "outside voices," using a few words in particular that would have curled sensei's whiskers. Mikey grinned.

"Ooh, I bet someone got into something they shouldn't have," he teased, poking Donnie playfully in the soft cartilage of his side. "What was it this time? Was it Raph's sais again? You know he just got those cleaned up from last time."

"It wasn't Raph's sais," Donnie mumbled. After a minute, he lifted his head and hit Mikey with those big brown eyes, and Mikey's whole heart just melted right there on the spot. "If I show you, you won't get mad, right, Mike? You promise?"

He could pretty much ask for the moon and Mikey would be like sure, absolutely. "I promise, dude."

That was all it took for Donnie to unfold his arms and show Mikey his prize—the dismantled mess of gizmos and wires and plastic casing that might once have been their secondhand toaster. Mikey bit the inside of his lip to keep from laughing, and only because Donnie's expression was so solemn and sincere.

"I just wanted to see how it worked," he said, totally disheartened. "I didn't mean to make Leo and Raph so mad. And I didn't mean to set the counter on fire."

Okay, the faraway shouting and commotion made a lot more sense now. Mikey gave the super-long purple tails of Donnie's mask a gentle tug. "Dee, you gotta get permission before you take stuff apart," he said seriously. "I don't care about the counter, or even the whole kitchen, but that fire could have hurt you, and then what would I do?"

Just the thought of it was kind of horrifying, and he did a quick once-over of his brother to make sure there were no new "boo-boos" anywhere on his little person. Donnie's head hung even lower at Mikey's mild reprimand. "Sorry."

Aww, Dee.

It was so hard to scold him for this kind of stuff, and not just because Mikey was terrible at the scolding thing to begin with. This kind of stuff was different, because Donnie wasn't getting into trouble to be ornery, he was just curious.

All the mishaps and dissected TV remotes—and, okay, the occasional fire—were just sort of the Donatello version of building blocks and ABCs. Mikey knew how smart Donnie was, way better than he did back when they were both little, and he knew that the broken toaster in Donnie's lap would be put back together better by the end of the week, without any instruction manuals or Google searches. Wasn't that kind of amazing?How was Mikey supposed to yell at him for that?

So Mikey hugged him instead, squeezing him until he squeaked a little. "I know you're sorry, buddy. It's okay as long as you didn't get hurt."

Donnie's little hands found his quickly, curling their fingers together. "I didn't get hurt. Leo dumped the soup on the fire before it could get big."

"Wait, he what? The whole pot?" Donnie nodded, and Mikey groaned. "But that was lunch!"

"You're complainin' about lunch?"There was Raph, looking pretty ruffled and thoroughly ticked off as he stormed over with a dirtied cleaning rag in hand. Donnie ducked back around his dismantled toaster, blinking owlishly at their red-banded brother from the safe circle of Mikey's arms. "You— I can't believe this. You're lettin' him get away with setting the kitchen on fire?"

"Oh, get over it," Mikey retorted with a roll of his eyes. "You've probably done way worse than that trying to make breakfast."

Raphael spluttered indignantly. "That's not the point! He needs to find safer stuff to play with!"

"By safe he means boring," Donnie mumbled, and Mikey rubbed his head.

He had a new appreciation for how hard it must have been on Master Splinter the first time they were growing up. Donnie kind of deserved whole libraries and laboratories and spaceships, he maybe deserved the entire world—and the best his family could give him was someone else's trash. Junkyard finds, dirty books and worn tools and broken things. It sucked.

"April promised she'd bring some of her school stuff for him this weekend," Mikey said, meeting Raph scowl for scowl in a way he never would have a week ago. "Until then, we're probably gonna lose a few things. And you can deal with it, 'cause whatever it is, Donnie's probably the one who built it in the first place."

Raph stared at him without speaking for a minute, and then his eyes fell to Donatello. The accident in the lab had happened nearly a week ago, and Raph hadn't warmed up to the new Don at all.

"He'll come around," was all Leo had to say on the subject. "And Rockwell says he can fix it. Don't worry."

So, fine. Mikey wasn't worried. But as Raph shook his head and turned away without another word, Mikey glared at the back of his shell.

"Jerk," he said, and didn't care if Raph heard him or not. Donnie was their brother, whether he was six years old or sixty, and treating him like a stranger was pretty much the worst. He heard a soft sound like a sniff from the little ball of turtle in his lap, and a good ninety percent of his irritation floated away like a plume of smoke. "Oh, you better not be crying, Donnie. What'd I tell you about crying?"

"If I do it, you'll do it, too," Donnie recited promptly, and his eyes were a little suspiciously wet when he looked up, but his face was dry, and there was a smile tugging at the corners of his mouth. "Thanks, Mike. And sorry about the soup."

"Not your fault, dude. Leo tends to panic around fire."

"Guess that means we have to order pizza, huh?" Donnie asked innocently. Mikey grinned outright at him.

"Tell you what, buddy," he said, bundling Donnie (and his toaster) up a little closer and rubbing their foreheads together. "I'm gonna miss you when you get big again, and go back to being too busy for me."

Donnie giggled at him, all soft and sweet.

"Don't be silly. You're the coolest brother ever! I'm never gonna be too busy for you."