A/N: A season 6 chapter. Spoilers for anyone who hasn't watched up until the holiday break.

Whoever said the apple doesn't fall far from the tree really didn't know much about parents and children, Oliver decided, or probably much about apple trees either, for that matter. It was one of those folksy sayings that sounded wise but didn't bear out in real life.

Because as much as he loved his son, there were times when William seemed to Oliver like an alien being. Yes, he had the beginnings of what was probably going to be a square jaw and he most definitely had a stubborn streak (especially on the subject of broccoli). His brooding skills, too, were first class. Oliver could relate to all of that.

But when it came to school, William was as different from Oliver as a salad was from a deep fried Twinkie. The kid worried about school. He cared about his grades and he studied hard - too hard, in Oliver's opinion. Oliver would have been happier to come home and find William binge-watching television instead of pouring over books. Neglecting homework was familiar territory; academic anxiety was not. It was frustrating for Oliver not to be able to connect with his kid over something important – especially when he was determined to be a more understanding dad than his own father had been.

Thank God for Felicity. From the moment Oliver brought her home to help William with algebra, he could see that they were two peas in a pod. (Which was another expression involving similarities and food, Oliver realized, only a vegetable this time instead of a fruit. Go figure.)

At any rate, the woman who shared no DNA at all with William understood his academic fears perfectly. She knew right away, for example, that telling the kid not to worry was only going to make things worse. And she knew William wouldn't be happy if she gave him the answers instead of helping him figure things out for himself.

And finally, she knew how to make Oliver see a silver lining in what felt like a cosmic Gotcha. Because Oliver Queen…the father of a serious student? With all due apologies to Alanis Morrisette, that really was ironic.

"It isn't just about grades with William – you get that, right?" Felicity asked Oliver one evening, over a late dinner in his kitchen.

"No," Oliver replied, "I'm not sure that I do get it. I don't know what his mother expected from him in school, but it feels to me like he's too focused on earning approval with good grades. I want him to like himself for himself – not because he aces a math or science test."

Felicity shook her head. "He's not studying to gain approval. If that was the only reason, he'd have stopped by now because you've made it pretty clear that you don't care about grades."

Oliver frowned. "Why else would he be studying so hard?"

Felicity patted his hand. "Because he's curious; he likes to know how things work and he's interested in the mysteries of the universe. Figuring things out gives him a sense of accomplishment."

Oliver had a hard time believing that any kid could be curious about school subjects. When he was William's age, the only mysteries he'd been interested in were hidden in Becky Lewinski's underpants.

"Seriously?" he asked Felicity. "What normal kid cares about math and physics?"

She stared at him and arched one eyebrow. He immediately realized his mistake.

"Well…okay…I mean, you cared about math and physics when you were a kid, but you weren't exactly normal…"

Her eyebrow arched higher. Oliver could see that he was digging himself in deeper. He paused to choose his next words more carefully.

"What I meant to say," he stammered, "is that you were exceptional. That's why you liked school. Because if you were just a normal kid who didn't like math and physics you wouldn't be able to do the amazing things that you do today. Which really are amazing. And which make me love you even more…"

She smiled. "Maybe William is exceptional, too," she said, not acknowledging his compliment. "After all, he is your son."

Oliver thought about that. Having a kid who was exceptional was a little frightening. Exceptional drew attention; it felt safer to have an unexceptional kid who could fly under the radar. On the other hand, having a son who cared about and worked for something other than his own pleasure was gratifying. Oliver believed that his own life truly began after Lian Yu, when he found a greater purpose. Before that, he'd just been a hedonist occupying space. If William was fortunate enough to discover some kind of calling early in life, Oliver decided he was going to be pretty damn proud of that.

"Fair enough," he sighed. "But can I count on you to help him with all the school stuff? Because honestly, when it comes to math and science, I don't know what I'm doing."

"Of course you can." Her voice was warm.

"Thanks." He took her hand and squeezed it gently. It felt so good to have her in his life again; a partner in all things, not just in the bunker. It was as if everything he'd ever lost had been found, all at once.

"Now," she continued, her voice light, "what was it you were saying about me doing amazing work?"

He looked at her. Her hair was loose on her shoulders and she was wearing red, one of his favorite colors for her. She didn't appear the least bit tired, despite the late hour.

"Let's go to bed," he suggested, "and I'll show you just how amazing I think you are."

She nearly sprinted to the bedroom.

Felicity helping William with his schoolwork turned out to be a good idea; better, even, than Oliver had anticipated. It allowed the two of them to form their own relationship, independent of him - a relationship founded on common interests and honesty; one they both clearly enjoyed.

Best of all, Felicity and William's friendship made William greet Oliver's eventual marriage to her with delight, instead of the jealousy often displayed by stepchildren. The kid had no fears about losing his dad's attention to a new wife. Probably, Oliver thought wryly, because William very often grabbed Felicity's attention first. There were evenings when Oliver rushed home from the mayor's office only to be greeted by half-hearted waves as Felicity and William kept their heads down over his books or continued playing a video game. Most nights Oliver could laugh about it. Occasionally, he wanted to wrestle the damn game console out of Felicity's hands, send William to his room, and remind her with kisses that she had two men in her life – and that he had seen her first.

One evening, however, turned out to have unexpected and lasting consequences. It started innocently enough – with a physics question from William – but ended up with a new worry for Oliver. And he really didn't need any more worries in his life.

Oliver arrived home late – after nine o'clock. He'd gotten stuck in a contentious meeting with the City Council and wanted nothing more than to have a bite to eat and go to bed (and snuggle with Felicity). He expected William to have already turned in for the night, but instead found his son and his wife sprawled in front of the television, with a half-empty pizza box between them and frowns of concentration on their faces.

"I don't understand what that guy was saying about space-time," William was saying to Felicity, gesturing toward the television. "Can you explain it?"

Felicity looked up at Oliver and pointed at the pizza box. "Welcome home. There's pizza left if you want some. It's veggie, so it has to be at least a little bit healthy," she added apologetically.

Oliver rolled his eyes. His wife was a joy in every way, but he wished she could embrace cooking a little more enthusiastically. If he worked late, he was sure to come home to one of two options - both of which were full of salt and fat; pizza, or Big Belly burger. It was fortunate that he, Felicity and William were all blessed with good metabolisms. Otherwise, it wouldn't have been pretty.

He took the remaining pizza to the kitchen, slid it onto a plate and popped it in the microwave.

"Felicity?" William repeated. "Can you explain the space-time thing?"

She sighed. "I don't know, William. It's college-level physics. And it's not very intuitive."

"Please? It was on the science program on TV, so they couldn't have thought it was too complicated."

She didn't immediately reply.

The microwave dinged, signaling that his pizza was hot. Oliver removed the plate and studied Felicity with interest. It wasn't like her to evade a question from William, especially one about science. He wondered why she was reluctant to answer this one.

"Felicity?" William persisted. The kid definitely had his dad's stubbornness.

She up on the sofa and ran her fingers through her hair. "Okay, I'll give it a try. But be patient. Like I said, it's not intuitive."

William nodded. Oliver took a bite of pizza and winced when the hot cheese hit the roof of his mouth.

"For thousands of years," Felicity began, "scientists believed that time was an absolute quantity, independent of everything else. Five minutes passing was the same for everyone everywhere; and now occurred at the exact same moment for everybody in the world. There was only one now – and then it became everybody's past." She paused and looked at William. "Does that make sense?"

William nodded slowly. "I think so."

"But what Einstein figured out," Felicity went on, "is that time isn't an absolute and independent quantity. It's bound together with space, and it passes differently depending on how fast we move through space. So, Einstein called the single, four-dimensional quantity space-time. And he told us that all those things about time that we believed for centuries and feel to be true in our gut are wrong. He told us that time is relative." She studied William's face for signs of understanding.

William looked confused. Oliver was right there with him.

Felicity tapped her fingers on her leg and thought for a few seconds. "Try thinking of it this way," she said. "Say I have two clocks that are perfectly synchronized. I put one of them on an airplane and fly it around the world a couple of times, and I leave the other one on the ground at the airport. When the plane lands back at the airport, the clocks won't be perfectly synchronized anymore. The one on the plane will be a few, tiny fractions of a second slower than the one the ground. Moving at six hundred miles per hour through space makes time pass differently. Which means that time isn't absolute – it's relative." She smiled. "They actually did experiments with atomic clocks on airplanes and proved this out."

William did not look impressed. Oliver was with him on that one, too. What did a few fractions of a second on an atomic clock matter?

Felicity glanced between them. "All right, I realize less than a second may not seem like a lot," she conceded, "but if the plane could move near the speed of light, the difference would be a lot more than a fraction of a second. Depending on how far the plane flew, the difference could be years. It's why scientists dream of having spacecraft that can move at light speed. Astronauts could travel huge distances and barely age. One year moving through space for them could be a few hundred years back here on earth."

William frowned thoughtfully. Oliver took another bite of pizza. Fat and salt notwithstanding, it tasted pretty good.

"Where this whole thing gets people wigged out," Felicity continued, "is that depending on where you are and how fast you're moving, time being relative means that what's happening during your now already occurred as part of somebody else's past…or is yet to occur in their future. There are physicists who have extended the theory to state that there really isn't a past, present or future – all moments exist simultaneously in the space-time continuum."

Oliver thought William looked a little cross-eyed at that one – which was understandable, because it was starting to sound like science fiction.

Felicity grinned. "Some people have even gone so far as to suggest," she finished, "that it means it's possible to change what we consider to be the past. If all moments in the space-time continuum exist simultaneously, then changing one part of the continuum might change all parts."

Oliver and William looked at one another.

"Is there any pizza left?" William asked. "I think that explanation just wore out my brain."

Felicity's grin faded. "I'm sorry. But you did ask."

Oliver carried what was left of the pizza over to William. His son took a bite and chewed slowly.

"But most of this weird stuff with time happens when you move near light speed, right?" William asked Felicity. "I mean, those of us on earth will never see it because no one can go that fast."

Felicity nodded. "That's right," she said soothingly.

William shrugged. "Then I'm not going to worry about it. I'll wait until I have to take the class in college to understand it." He got up and gave Oliver and Felicity a short wave. "Goodnight," he said. Then he headed for his bedroom.

Oliver gave a silent cheer. It was a little before ten and he had Felicity to himself. Plenty of time for a little X-rated snuggling before he had to fall asleep and then get up in the morning to face the City Council again. He saw the grin return to her face and knew they were on the same page.

It wasn't until they were both in bed and he lay poised over her, working his hand under the pajama top she had needlessly taken the time to put on, that something occurred to him.

There was a person who could move at the speed of light – faster, in fact. And he'd already altered the past at least once. He had made John Diggle's daughter - along with everyone's memories of her – disappear.

Barry Allen.

Barry I-Can-Mess-with-the-Space-Time-Continuum Allen.

Shit.

Oliver's hand came to a stop as he considered the implications. When Barry had first told him about John's daughter it had been a shock – but an abstract sort of a shock. The same sort of shock Oliver felt when he saw an earthquake or a plane crash on television. It was terrible, but it wasn't personal. As a supportive friend, Oliver had voiced his sympathy to John but he hadn't spent hours dwelling on the fact that Baby Sara Diggle was no longer in the world. After all, John had ended up in a good place – happily married with a son. And Oliver had no memory of Sara. It was tough to mourn a person you had never known.

But Felicity's explanation about space-time made Oliver realize that Baby Sara disappearing wasn't a fluke. Barry Allen had changed the past once and there was no reason why he couldn't do it again. Hell, maybe any time the sonofabitch went racing around at light speed he might inadvertently make something go away. And the world would be none the wiser.

Oliver looked down at Felicity – at the woman he was finally able to call my wife after six years – and felt a shiver run down his spine. He had wasted two of those six years not admitting that he loved her; another year thinking they couldn't be together; and the rest of the time in a screwed-up comedy of errors filled with lies and substitute relationships. They'd finally gotten to where they belonged – where they should have been all along. And Barry Allen could undo all of that simply by flexing his speed muscles. He could create a world where Oliver might lose Felicity…or even a world where Felicity had never existed, just like there was no longer a Baby Sara.

Shit.

"Oliver?"

Felicity was looking up, studying his face. She was smiling, but there was a hint of confusion in her eyes. He moved his hand to her cheek and stroked it lightly with the back of his fingers.

"Oliver, what's wrong?"

He shook his head, not wanting to ruin her night. "Nothing's wrong," he lied. "Just thinking about how lucky I am." He lowered his head to kiss her.

"Bullshit." Her lips moved against his as she said the word. "That's not an I'm lucky face. That's an I'm worried about something face. So, I repeat, what's wrong?"

He should have known he wasn't going to get away with trying to distract her. She could read him far too well. Oliver shifted his body so that he was lying on his side close to her. Then he wrapped one arm around her waist and rested his head next to hers on the pillow.

"I was thinking about your explanation to William," he said quietly in her ear, "about space-time and the idea that one change in the continuum might result in other changes – even to things that occurred in the past."

She frowned. "You were thinking about physics just now? When you had your hand up my shirt?" She grimaced. "I must be losing my touch."

He chuckled, despite himself. "Of course not – you could never lose your touch when we're together like this. It's just that…" He paused, searching for words. "It's just that I don't want to lose you, Felicity. We've had so many things stacked against us and we've finally made it to where we should be. The idea that our history could change – that something could change it - well, it scares me. We could end up separated, through no action of our own. We could even end up in a world where we've never met. And I don't want to ever be without you again."

He thought she might laugh at his foolishness, but she didn't. She rolled onto her side to face him and rested one hand lightly on his chest. "The chances that we get separated due to a ripple in the space-time continuum are pretty remote, Oliver," she said. Then she smiled. "It's more likely that you'll get angry with me because I keep giving William junk food for dinner."

He didn't smile back. He wasn't going to allow himself to be distracted either. "You know that's not going to happen, Felicity." He put his hand over hers. "But let's face it; neither one of us exactly leads a regular life. We've seen some pretty weird stuff. Mirakuru soldiers, aliens, a Lazarus pit that brings people back from the dead—"

"My mother and father dancing together at our wedding reception."

He shushed her gently with a finger against her lips. "Seriously - I wouldn't put it past the universe to mess with us by changing the past. And we both know someone who can move at the speed of light."

She sighed. "Barry."

"That's right. Barry."

"Hmmm."

He was quiet for a few moments.

"Is that why you were reluctant to explain space-time to William?" he asked at last. "Because he knows The Flash can run at light speed?"

She nodded. "I thought he might put two and two together and come up with some difficult questions. He's a worrier, like you Oliver, and he's a smart kid. I'm actually a little surprised he didn't go there."

Oliver gazed at her disheveled, glossy blonde hair. "I'm relieved that he didn't."

She tapped his chest. "No…you went there instead. And now you have worry-face. Which might be worse."

"Sorry."

She leaned forward and kissed him on the underside of his chin. "Well, look on the bright side, Oliver. If our pasts do get changed, it most likely won't bother us because we'll never know the difference. We'll be living altered lives; we won't ever have met."

He wrapped his arms around her and pulled her closer. "I think that would be the worst thing of all – to never have met you."

"Really? You don't think ignorance will be bliss?"

He raised his eyebrows. "A world where our lives don't intersect? Where I never get the chance to know you?" He shook his head. "I don't want any part of that. You've changed me in so many ways. No - I'd rather remember you, and if I have to, deal with the pain of losing you."

"Tis better to have loved and lost than never to have loved at all?" she murmured into his chest.

"Exactly." He ran his fingers through her hair. "Shakespeare?" he added.

"Tennyson, Oliver. Tennyson said that."

He sighed. "Oh…Tennyson - right." He shrugged. "Well, we both know I wasn't the brightest student. Unlike my son, apparently, who is much smarter than his old man."

She shook her head. "You're plenty smart, Oliver. Look at all the things you do well today. You're the mayor of Star City and you have the respect of people like John Diggle and Quentin Lance. You just weren't motivated when you were in school. Having watched you these last six years, I believe you can do anything when you're motivated."

"I can?" He felt a small rush of pride.

"Absolutely." She began to caress his chest with light strokes. "And right now, I think you should be motivated to take care of my needs."

"Your needs," he repeated.

"Yes. I'm a grown woman and I have needs." She moved her hand lower, from his chest to his abdomen.

"And it's my responsibility to take care of these needs."

"It is," she stated. She moved her hand lower still, past his abdomen to…

Oliver suddenly found himself very motivated.

"Well, I wouldn't want to shirk my responsibilities," he said, "in any universe."

A few days later Felicity sat surrounded by paperwork, feeling both excited and a little frustrated. The papers were the documents that legally established her start-up business, Helix. The excitement came from the challenge of standing on her own two feet and doing work that she loved. The frustration came from a dislike of all things legal and bureaucratic. She placed the stack of papers in front of her and reluctantly started reading.

She was on her fourth whereas when her phone rang. She was surprised to see that the call was from Barry Allen.

"Barry?" she answered.

"Hey, Felicity," he said.

"To what do I owe the honor? Is everything okay?"

"Everything here is fine," he said, "well, at least as fine as it ever gets. I was calling to ask you the same question. Is everything all right with Oliver?"

Felicity frowned, instantly concerned. "Yes," she said nervously, "as far as I know. Why?"

"He called me yesterday," Barry explained, "and told me that I'd better be - and these are his words – damned careful about screwing with the space-time continuum. And then he basically hung up. So I was wondering if you knew what prompted the call."

Felicity smiled, relieved. She should have known that Oliver wasn't going to let this one go. "Yes, I'm pretty sure I know what prompted it. Don't worry – it's nothing bad. Just some fallout from a physics lesson with his son a few days ago."

"A physics lesson? He sounded serious, Felicity. He was using his Arrow voice."

Her smile grew broader. "I'm sure he was serious. But he's not going to do anything – I promise. I'll talk to him tonight."

"Okay." Barry didn't sound entirely convinced.

She gave him a few seconds. When he didn't add anything, she said, "Was that it? Or is there anything else on your mind?"

"No," Barry replied, "that was pretty much it. I guess we'll just keep in touch."

"Sounds good. And Barry?"

"Yes?"

"It probably would be a good idea to not screw with the space-time continuum. You know, just in case."

Felicity hung up before he could ask any more questions.