The first time it happened, Tony was completely unprepared.

All of the Avengers except Thor, who visited whenever he could, lived at the tower. They each had their own floor that had a bedroom, multiple bathrooms, a living room, kitchen and training room. Tony was up on his floor when the first thunderclap shook the sky. He jumped a little, then scoffed at himself. It was just thunder.

He turned over in his bed and told JARVIS to open the blinds so that he could see outside. He watched the rain go from a drizzle to a downpour in only a few seconds. Every time lightning flashed across the sky, he grinned and every accompanying BOOM! made his heart beat a little faster. He loved thunderstorms.

Bruce, as it turns out, didn't. Not long after the fifth thunderclap, Tony heard pounding at his door. He frowned at the ceiling.

"JARVIS, what time is it?" he asked.

"It is three seventeen in the morning, sir," the AI responded. The knocking came again, more insistent this time, and Tony sighed. It's not like he was actually sleeping, but he still hated being disturbed super late. Reluctantly, he pulled himself out of his warm bed and trudged to the door. It opened smoothly when he pushed a button on the control panel and Tony was surprised to see Bruce there.

He looked scared. No, not scared, terrified. His eyes were wide and glittering with wetness, his hair and clothes were rumpled and his hands shook badly.

"Bruce?" Tony said, frowning at the appearance of his best friend. "What's wrong?"

"Um," Bruce started, suddenly looking sheepish, but he didn't get to continue as another crash of thunder drowned out his words.

Tony grinned.

Bruce didn't.

He jumped and flinched horribly, more terror lighting up his brown eyes, and then Tony understood.

"Are you afraid of thunder?" he asked. Bruce nodded, looking on the ground and looking very much like a child. "Well, what do you want me to do about it? I'm not Thor, I can't control the weather."

Bruce's eyebrows furrowed and he opened his mouth to speak, but a flash of lightning lit up the hallway and his eyes widened even more. He stepped closer to Tony just as the thunder roared overhead, and flinched again. Tony had only ever seen Bruce look so afraid once or twice, and each time the Hulk had taken over. Suddenly fearing for the safety of his tower and its occupants, he decided to calm Bruce down.

"Bruce," he said, "thunder can't hurt you. It's just a sonic boom created by the displacement of air by a lightning bolt."

"I know that," Bruce muttered, still not meeting Tony's eyes.

"Then what's the problem? It can't hurt you, so there's nothing to be afraid of, it's simple."

"I know. It's just… I had a bad experience when I was little, and everyone else is asleep…"

And Tony knows what Bruce needs, and it isn't a scientific explanation for thunder.

"Do you want to sleep in my room tonight?" he asked softly and Bruce's eyes shone gratefully at him. He stepped aside and let Bruce in, leading him to the bed. He rolled his eyes when Bruce hesitated, reaching out and pulling him onto the bed next to him. He pulled the blankets up and then asked JARVIS to close the blinds.

Bruce was tense at first, but then another peal of thunder rippled across the sky and Bruce scooted closer to Tony, sliding down on the bed until his head lay near the billionaire's chest. Tony frowned, but didn't say anything.

"Thanks, Tony," Bruce mumbled, and Tony can hear how tired he is. He wraps an arm around Bruce's waist hesitantly.

"No problem, science bro," he said. Bruce sighed softly and wiggled closer until his head was lying nearly on Tony's chest. When the next crack of thunder split the sky, Bruce didn't flinch, just tense momentarily. He eventually relaxed and fell asleep against Tony, and the billionaire had no idea what to do.

He's used to being able to toss and turn as he pleased while in bed, but he doesn't want to wake up Bruce. The man looked like he hadn't slept for days, which he probably hadn't. He and Tony both had a bad habit of staying up far longer than healthy in the name of science.

"You're such a big baby," he whispered softly, and Bruce just mumbled something under his breath. Tony smiled and looked towards the covered window, waiting for the sun to rise.

...oO()Oo…

The second time it happened, Tony was surprised that Bruce had come to him again, but not unwilling to let his friend sleep with him. Around midnight, he let Bruce into his room and fell asleep with Bruce's head on his chest.

The third time it happened, Tony was sort of expecting Bruce to show up, and he wasn't disappointed. Bruce didn't even have to say anything anymore, just show up and Tony was ready to let him in. Neither of them were able to sleep that night, but Bruce managed to keep his heart rate down and his head on Tony's chest all night.

The fourth time it happened, Tony checked the weather in the morning and had JARVIS automatically let Bruce in when he approached the door. Bruce's head ended up- where else?- on Tony's chest.

Notice a pattern? Yeah, so did Tony.

So the fifth time it happened, he kept careful documentation of everything that happened prior to Bruce falling asleep.

Bruce showing up at eleven at night? Check.

Bruce sliding into Tony's bed with no amount of awkwardness whatsoever because he had slept with Tony so often that it might as well be his bed, too? Check.

Bruce flinching at a thunderclap and laying his head on Tony's chest with his ear pressed against the arc reactor? Check.

Wait.

...oO()Oo…

"Bruce?" he whispered the sixth time it happened.

"Hmm?" Bruce hummed sleepily back. He was laying on his side, head on Tony's chest, as usual, and the rest of him curled up into a little ball.

"Why do you always use me as a pillow?" He knows, he just wants to see if Bruce will admit it.

"It's the arc reactor," Bruce mumbled sheepishly and Tony smiled in the darkness. "The glow and the hum, it's soothing. I can't sleep during a storm unless I can hear it." Tony chuckled and used both arms to hold Bruce close.

He debates making another arc reactor and shoving it into a stuffed animal or something for Bruce to listen to whenever there's a storm, but it would be much too much work and money. Besides, he rather likes having Bruce in his bed, even if it's only when it rains too hard. Maybe in the future it'll become an every night thing.

Tony closed his eyes. He'd like that.