Bruce Banner was a physicist, and physics had quite a lot to say about time travel. Unfortunately, most of those things were best filed under the heading "it's impossible, and here's why", which he found quite infuriating given the very real evidence to the contrary sitting in the next room.
Bruce rested his chin on his left hand, pen hovering over a page full of scrawl, and listened. The details of the conversation from the next room weren't entirely clear, but both participants sounded happy enough. The childish chatter continued almost without pause, only occasionally interspersed with the attentive tone of JARVIS answering the boy's questions.
Tony's questions, Bruce reminded himself, because there was no doubt in his mind that the little boy currently playing with the pile of sensors, motors and double-A batteries was Anthony Edward Stark, aged four and five sixths.
Bruce sighed, looking once more at his notes. He had considered been the possibility that the cause was chemical or biological, but had dismissed the theory quickly. Even a cursory examination of the boy had shown no arc reactor embedded in his chest, and there was an obvious lack of memory beyond his biological age. The only conclusion Bruce could accept was some form of time manipulation.
Or magic, he supposed. But Bruce didn't like that idea. Magic was cheating.
Bruce was startled from his reverie by a sound very like a dog's bark, immediately followed by a squeal of childish delight. He rose cautiously to investigate.
He'd obviously expected Tony to take quickly to the collection of child-safe electronics components he had scavenged from the the Stark labs. They were the closest thing to children's toys that he'd had been able to gather at short notice, but Tony had seemed more than pleased with his bounty.
So it was true that Bruce had expected Tony to present him with blinking LEDs, perhaps something rolling around on the little rubber wheels they'd found. He most certainly had not anticipated the five inch high robotic... animal... which was currently lumbering around the floor. It had four legs sticking out like rods from a lumpy body, and a solid round head with paper ears sticky-taped to it. Despite the odd appearance though, it was successfully (if awkwardly) navigating its way around a makeshift obstacle course of chairs and boxes spread across the floor.
The robot creature barked. A dog, then. Tony gave another delighted laugh. Bruce stared.
"Did you make this, Tony?" he asked, trying to keep the surprise in his voice at a reasonable level. He looked to the little boy, the child's eyes bright with pleasure. Tony nodded.
"Uh huh. JARVIS helped though, 'cause he found me a sound for the barking and showed me how to put it into the... um..."
"Audio synthesiser, Sir." JARVIS contributed thoughtfully. Tony grinned and nodded enthusiastically.
"Yeah, the audio thing. So now he can bark!"
Bruce watched the dog approach a dead-end, blocked in on all sides. The dog sat down on its virtual haunches and gave another bark. Tony scrambled to his feet to rescue his creation.
"He can't turn all the way around yet, because I could only find joints that turned around this much." Tony demonstrated a right angle by extending his thumb relative to his fingers, "But JARVIS said he would get me some better ones tomorrow."
Tony returned, carrying the robotic dog with a hand under its belly. The dog's legs wiggled frantically for purchase. "If you pat him, he wags his tail!"
"This is very impressive, Tony." Bruce enthused, not even bothering to hide just how impressed he was. He reached out to pet the dog's head between the paper ears. Its tail did, indeed, wag. Bruce smiled. "You know, I used to have a pet dog."
"Really? What was its name? Was it a girl dog or a boy? Where is it now?" Tony asked, then his face suddenly turned sombre and serious, eyes wide with sympathy beyond his years. "Did it die?"
The child's eyes suddenly reminded him so fiercely of an adult Tony that Bruce couldn't help himself. He reached over to ruffle Tony's hair affectionately, but Tony twitched away from his hand. Bruce was momentarily startled by the reaction, but of course he was still little more than a stranger to this Tony. As much as the older version actively sought out Bruce's touch, it was to be expected that the younger would be a little nervous. He offered Tony a smile instead, allowing the boy his space.
"No, my dog didn't die, but I had to move house and he couldn't come with me." Bruce answered. He hoped that was true. He hadn't been able to go back to check on the poor animal after the military had hunted down his hiding spot all those years ago. He had had good neighbours at the time though, he hoped they would have taken the creature in.
Tony seemed to take the explanation at face value, his gaze falling thoughtfully to the odd little robotic animal which was currently lying down. Its tail tapped the floor intermittently. Tony looked back up at Bruce for a moment, lips pursed, before breaking out a shy smile and picking up the dog with both hands.
"You can have this one instead," He offered, holding the robot up. His eyes darted from the floor to Bruce's face and back again, battling between his shyness and his desire to observe Bruce's reaction. "It's not a really truly alive dog, so it won't mind if you ever have to move. You can take it with you."
"Oh, Tony. You made this for yourself, you don't need to give it to me." Bruce replied gently. He regretted it barely a second later, as Tony's face fell, unable to satisfactorily mask his disappointment. "It's very well done though, I'm very impressed." he added, trying to mitigate his error.
Tony sighed, the gesture exaggerated by his tiny, skinny shoulders. He put the dog down, flipping it onto its side and pushing it away with his foot. He turned his attention to a small pile of pre-cut wires to his left instead, picking each one up in turn and straightening it out before placing it back on the ground.
"Hey, is something the matter? Are you okay?" Bruce asked gently. He felt a powerful desire to pull the little boy over to him, but Tony's earlier flinch was still fresh in his memory. Tony nodded, looking up from the wires with a smile which held too many teeth yet completely failed to reach his eyes.
"I'm fine, thank you for asking. But if you're very busy then you don't need to stay here. I'm okay with JARVIS."
Bruce was at a loss. For all that the child's genius clearly matched the adult Tony's, the child's softly spoken and obliging nature was... Unexpected. But it was certainly true that JARVIS had been doing a stellar job of babysitting so far. Much better than Bruce was currently doing, not that that would be particularly difficult. He stood.
"Okay, but I'm just next door. So if you need anything, or if there's something you want to show me, you just come get me, okay?"
Tony gave another empty smile and nodded, before turning back to his collection of wires. Bruce watched for a few moments, before quietly returning to his desk.
He had been there for less than a minute before JARVIS interrupted his thoughts, the english voice pitched softly enough not to carry into the next room.
"Master Banner, your presence is required urgently. Master Tony is becoming increasingly distressed."
Bruce was up and moving before JARVIS had even finished speaking. He was far from prepared for what he found.
Tony was sitting in the middle of the room, pulling viciously at the defenseless robot's legs, twisting them off. The recently straightened wires, along with the batteries which had been powering the creature, were scattered across the floor as though they had been thrown.
"Tony, what are you doing?" the first leg came off, and Tony started work on the next, "Hey little guy, you don't have to take it apart just because I..."
Tony was barely listening, his attention focused on destroying the toy with an anger that seemed to have come from nowhere. Bruce reached over to place his hand on Tony's wrist in an attempt to still him before he did irreparable damage to his creation.
It was the wrong move.
Tony dropped the robot immediately, trying to pull his arm from Bruce's grasp, then went very, very still. If it wasn't for the fact that Bruce could see the small, almost panicked rise and fall of Tony's shoulders as he breathed, he might have thought the child had turned to stone. Bruce released Tony's wrist like it was burning, but not before he noted the racing pulse. Tony scooted backwards, away from Bruce, accidentally kicking the two-legged robot still lying on the floor.
"Hey, it's okay Tony. I'm not going to hurt you. I was just worried about your puppy, that's all," Bruce soothed. "Are you okay? Does your wrist hurt?"
Tony eyed Bruce cautiously, his breathing ever so slowly evening out. His eyes were bright with carefully controlled moisture. He shook his head. "No, sir. My wrist is fine. I'm sorry."
"No need to apologize, Tony. You haven't done anything wrong." Bruce once again quashed his desire to reach over and pull Tony into a hug. He was quite certain now that such a gesture would not be well met. "Hey, how about we fix up your puppy, then we'll go show it to Steve. You remember Steve? I bet he'd get a real kick out of a robot dog."
Tony continued to stare at Bruce for several seconds, before scooting back across towards the robot and carefully reattaching one of the legs which he had so roughly removed only moments earlier. He shook his head in response to Bruce's comment though. "I don't think he would like it." Tony explained.
"Why do you think that?" Bruce prompted, curious. Tony shrugged.
"I just don't think he would. He's Captain America."
"Yes, I know he is. But I also know for a fact that he gets pretty excited by new inventions and robots and things like that."
Tony attached the last leg, then gathered up the loose batteries. He placed them on the floor between them, an offering.
"Maybe you could take it to him then. I'll just wait here with JARVIS." Tony said, eying the dog with sadness.
"I'm sure he'd like it even more if you showed him yourself. Why don't we go up together? Or maybe JARVIS could invite Steve down here?" Bruce suggested, hesitant to leave the little boy unattended again so soon after his unexplained meltdown. The suggestion only served to make Tony more nervous.
"I'd rather not, if that's alright?" Tony fidgeted with the fabric of his shirt. Bruce sighed inwardly.
"Of course it is. You don't have to show anyone unless you want to. But maybe we could go for a walk anyway? Just you and me, we can go down to the gardens and have a look around."
Tony pursed his lips. "Can JARVIS come too?" He asked.
Bruce smiled warmly.
"Why don't we ask him? JARVIS, would you like to join us in the hydroponic gardens downstairs?" Bruce's eyes never left the little boy in front of him.
"It would be my pleasure, sirs."
Tony smiled, small but real this time. "Okay then." He stood, brushing loose pieces of wire from his clothing. Bruce stood as well, holding out his hand. Tony looked at it suspiciously for several seconds, then tentatively took it.
Bruce led him gently to the door, and pretended not to notice the way Tony stared at the place where their palms touched.