A/N: This has been returned to a one-shot. Sorry. The "In Progress" status was haunting me. At least now it's not left hanging and feels like it has an end. Although if anyone wants to adopt it, that's fine by me. Just send me a PM.

A Weak Constitution

Don rapped his knuckles against the half-open door as he stepped into his brother's office. "Charlie?" The familiar staccato clacking of chalk continued uninterrupted. Peering around at all the clutter, he still couldn't quite spot the source. "Charlie?"

Out of the corner of his eye, something shifted. The agent swung his head toward it, confused for only a moment before he spotted his brother's legs behind a free-standing chalkboard. "Hey, Charlie!" he called in exasperation. Picking his way through the mess and squeezing past the board, he finally spotted his brother, ensconced in the corner.

Charlie jumped and dropped his chalk. Yanking off his headphones to hang around his neck, he exclaimed, "Don! Don't do that!"

Don threw his hands up. "Sorry!" Letting them drop to his sides, he shifted his weight, propping his right shoulder against the nearby wall. "But Dad was wondering if you were ever going to come home tonight." Glancing around, he almost didn't want to ask, but... "Why are you hiding back here?"

Charlie straightened from gathering up the fragments of his chalk. Setting the pieces on the board's ledge, he muttered, "I'm not hiding."

"Sure." Don raised an eyebrow.

Charlie relented. "Okay, well... maybe a little." He winced.

"What's up?"

His little brother studied his shoes a moment, sighing. "I've almost got this algorithm worked out," he gestured to the board, "and I hate interruptions." He glared at Don, before his face slid into the "other brood." Charlie sighed again. "But... it's Marshall Penfield."

Don straightened. "That guy who went to Princeton with you?"

Charlie nodded. "Yeah, I think he's here." He ran a hand through his curly hair and sent a worried look at the equation on the board. "He's going to tear it apart! I'm not even done with it yet!"

Don didn't quite know what to do about that, but he was here on a mission. "Okay, well, Charlie, it's really late, and even your geek brain needs rest." Don gripped his brother's shoulder. "Penfield can't do anything this time of night. Just work on it tomorrow."

Charlie shrugged him off. "No. It's almost done." His tired eyes took on a nearly feverish gleam as he grabbed a fresh piece of chalk.

Don made a face and fought off a yawn. "Come on, Charlie. It's almost two in the morning. Give it a break."

"Not yet."

"Damn it, Charlie! You've been working on this thing all week!" He threw up his hands. Even if he did manage to drag him home, knowing Charlie, the stubborn professor would just keep working on it in the garage. "I give up. I shouldn't have even driven out here." He was mid-way through turning away when a dry cough from behind stopped him in his tracks.

Charlie stood frozen, left hand over his mouth, staring at his brother like he'd been caught reaching into the cookie jar. His hand fell slowly to his side. "It's just chalk dust," he protested.

"Charlie," Don began worriedly, drawing out the syllables.

His little brother's face tightened mutinously. "Just chalk, I said."

Don narrowed his eyes and stomped toward Charlie, snatching up the hand curled at his side. Over his brother's protests, he pried open the fingers and scrutinized them. Don then turned again and tugged him along by the wrist, commanding, "Time to go home, Charlie."

"Don! Don! Let go!" Charlie jerked on his hand.

His older brother growled over his shoulder, "I'm not going to stand by while you make yourself sick again." He shoved the blackboard out of his way, knocking over a pile of books in the process. "Last time was bad enough," he murmured to himself.

Charlie staggered along less reluctantly after that, barely managing to deposit the second piece of chalk on his desk on the way by. Don finally let go of him when they were a dozen yards away from his office door. Charlie glowered at him, jerking his clothes straight. "I can take care of myself, Don."

"Yeah, well, apparently you can't." Don folded his arms.

"I'm not a kid anymore."

Don's brow furrowed. "You act like one! Do you want to get sick?" His patience never lasted very long when he was tired.

"Of course not." The professor's gaze slid guiltily to the side.

Don sighed and released his anger. "Come on. Dad's still waiting to hear whether you've collapsed or not."

"Hey!" Charlie brandished an affronted finger. "That only happened twice!"

His older brother's face sagged into weary lines as he huffed a laugh. "Twice is plenty, Buddy. Twice is plenty." His gaze went distant with remembrance of past anxiety.

Charlie scratched the back of his head. "Sorry."

Don returned back to the present with a nearly visible mental shake. He gently bumped shoulders with his brother. "Yeah, don't worry about it."

Charlie sighed. "Not sick," he muttered petulantly.

Don lifted an eyebrow. "Not if you get in the car and let me drive you home. I need some shut-eye, too, you know."

"Oh. Right. Sorry."

Don shook his head, smiling. "Let's just go home, Buddy."