"Holy half-baked shitballs," Rin continued cursing. She threw her hands up during her disrespectful diatribe and let her derrière descend on a nearby deck chair. "Holy shit, holy shit, holy shit!"

"Does she do this often?" Martin asked Darla, who only shrugged as they watched Rin gibber, her voice trialing off even as her mouth kept moving.

"I think you broke her," BIRD said.

"What?" Martin asked. "How did I break her? She told me to say that to her."

"Yeah, she told you tomorrow," BIRD explained with all the patience of a tech-support employee on his last day of work. "But today, it means she just proved that you met her in the future, then travelled back in time. For a theoretical physicist working on the possibilities of time travel, this is a lot like a theologian finding proof that God exists."

"I didn't lose my shit like this when I time-travelled," Martin protested.

"That's because you're too stupid to see the implications of what you did," BIRD said. "For you, having that stupid macguffin in your pocket is just a convenient plot device. For someone who actually uses their brain, a working method for time-travel is like knowing there's a button to end the universe. For instance, what happens if you cause a paradox?"

"What's a paradox?" Martin asked.

"What?" BIRD's eyes widened, and it took a step back, trying to get as far away from Martin's head as his shoulder would allow. "Please tell me you're not serious."

"Gonna order a pint of battery acid just so I can dunk you in it," Martin warned the little robot.

"Holy shit. You really don't know what a paradox is," BIRD said. "This is why government's can't scrimp on public education. Look, a paradox is a contradiction, like when two things are somehow true when they're mutually exclusive. Like you being able to walk and talk, despite being as astonishingly stupid as you are."

"Barkeep, a pint of the strongest industrial acid you have," Martin said.

"What the little robot means, we don't know the consequences of travelling back in time," Rin said, and she stepped up to Martin. "Darla, Craig, I think we're okay. This isn't one of those problem guys I keep getting."

"You sure, Rin?" Darla asked. "Was kinda looking forward to this one. I'm honestly not sure Craig and I could take him. Guy's pretty dangerous."

"Really, it's okay. And thanks again."

Darla and Craig both pushed-off the bar and took their formidable bulk back to a nearby table. Martin watched them both leave, and let out a slow sigh of relief as he felt he could finally relax. "So I'm not the first guy who's ever tired to tell you he met you in the future?"

"Some desperate guy hoping to 'get to know me better' by claiming we already met? It happens every other week," Rin admitted. "Often enough we had to work out this routine to deal with them. It's almost as bad as being a romance writer on Wattpad, and getting DM'd by Hi-Boys every other day."

Rin gestured to a nearby table. "Come on. Let's sit, so you can tell me about travelling backwards in time without an audience."

Martin's old training kicked in, and he looked around, weighing his options. He eventually settled on one table in the far corner of the room, near the back exit. It had no windows, and if he sat with his back to the wall, he could keep the entire room in his line of sight.

Except, he realized as Rin was now halfway across the room, he wasn't the one picking the table. Martin caught up only as Rin sat down at a table in the closest corner, near two windows so large Martin could drive his bike through either one. It was only very reluctantly that he sat down in the seat across from her and waited.

Rin sat down across from him, shifted into that unnaturally upright posture popular among academics, nobility, or other forms of unwarranted snobbery. "So, a paradox is a kind of contradiction, one that involves two or more things that should prevent the possibility of the other. A popular example is saying 'this statement is false', or claiming that caffeine makes you sleepy. In time travel, it would be a continuity paradox. Like what would happen if you went back in time and killed your grandfather before you or your parents were born?"

Martin blinked, and shook his head. "What would happen if I did that?" he asked.

Rin waved her hands in the air, and then put them both in front of her mouth, stifling a scream. It took her a moment to calm down enough to just yell at him. "You tell me! You're the one who's actually time-travelled!"

She coughed, and took a sip of water in the way academics often do to stall for time. "Up until this conversation, every worry about that kind of paradox was speculative. We couldn't try any of it. There was no reason to wonder what might happen if you went back in time and killed your grandfather, or Hitler, or the original protozoa all life is descended from. Because it couldn't happen."

"Okay..." Martin muttered, his eyes widening.

"It's something most people have always wondered about time travel. For instance, if you did go back in time and try to kill Hitler, did that attempt you made already happen before you went back in time?" Rin asked.

Martin blinked, and shook his head. "I'm afraid you lost me."

Rin smiled, the way someone might smile at a toddler trying to tie their shoes. "Imagine yourself with whatever machine you used to travel back in time with. You've decided you're going to pop back to Germany, circa 1910, and blow Hitler's brains out before he can lead Nazism to some of the worst crimes humanity ever committed. Now, at the time you decided to do that, Hitler, Nazism, and World War 2 all definitely happened. Is that because you haven't travelled back in time yet?"

Martin gaped, uncomprehending.

"It's okay if you don't know. The rest of us don't, either. You see, it might be that your attempt to kill Hitler already happened, and it didn't work. Or it does work, but everything else happens exactly the same way, only someone else become leader of the Nazi party. Or perhaps it does work, and everything changes. What happens to the world you knew before you travelled back in time? And what happens to you?"

"Wait, do you mean if I went back in time and changed things, I could change things so that I don't exist?" Martin asked. "But how would I have changed things, if I didn't exist to go back in time and change them?"

"That's why it's a paradox. Perhaps you'll continue to exist after you commit the deed, as long as you don't travel back to the time you left. Perhaps you'll stop existing as soon as you do it, in which case, how could you have affected the past when you stopped existing before you travelled back in time? Or perhaps you'll change the future, but still be able to return to it because you existed in the past before you were actually born. Up until now, it was all hypothetical, because it was impossible."

Martin's thoughts were incoherent, fragmented, like the operating system for this thoughts had encountered a system error and crashed. He was spared the admission, however, when the barista came to his table and set another cup of espresso in front of him.

Martin looked up, surprised to see the man looked strangely giddy. He could barely keep still; kept shifting his weight from one foot to another, wrung his hands, had his eyes open unnaturally wide, and had his mouth open as if he needed to pant because his head was overheating from all the excitement. "I tried it your way, mister mercenary, sir."

The barista kept hovering nearby, apparently waiting. Martin picked up the cup, stirred it with a small spoon, and tried a sip. His eyes widened, and he smiled as he leaned back. "That's better," he admitted.

"Just better?" the barista asked, loudly, incredulity dripping from his tone the way a bit of drool now fell from the corner of his mouth. To Martin, it looked like the man was torn between being indignant and impressed.

"Congratulations, it tastes like coffee is supposed to," Martin replied. "You waiting for a medal, or something?"

"Are you serious?" the barista asked. "That might be the best tasting shot I've ever poured."

"Wait, you didn't take a sip of my coffee before you served it to me, did you?"

"How else was I supposed to know it tasted right?"

Martin was reaching for his sidearm, and only stopped because Rin leaned forward and extended her arm. "Gimme," she said. Martin handed her the cup, and she put it to her lips.

The only reason Rin didn't drop it, was because she gripped it tighter and downed the rest in a single gulp. "Holy shit," she said, before she stood up, swung her arm, and smashed the tiny cup on the ground. "Another!" she cried.

The barista nodded and turned away, and nearly made it a step before Martin caught him by the arm. "Get me another, as well. And if you take a test-sip from mine, I will feed you my BIRD drone."

"What did I do?" BIRD asked.

"Besides insult me with every other sentence?" Martin retorted.

BIRD shook its little head. "Yeah, fair."

"But back on the most important topic," Martin said. But he pointed to the shattered cup at Rin's feet. "Why did you do that?"

"Oh, around here, it's easier to have the cup melted down and rebuilt in an automated 3D printer than it is to wash it," Rin explained.

"Seriously?" Martin asked.

"Seriously. 3D printing tech is extremely efficient, but getting water shipped here costs an appalling amount of money. Luca Cardego is a generous landlord, but as a businessman he's more ruthless than Vlad the Impaler. This place might look like a lush cityscape, but when it comes to our water usage we're one missed shipment away from being Arrakis."

As Rin explained, a little round robot scurried over, swept up the broken cup, and drove back out of sight.

"Wait, sorry, Arrakis?" Martin asked.

"Dune. You know, the sci-fi novel that was basically Afghanistan in 2021, when a band of fundamentalist religious zealots threw out the universe's most powerful empire and the leader felt it was appropriate to have multiple wives," Rin explained.

Martin blinked, and hesitated before he said anything. "I am deeply uncomfortable with that parallel."

"So is the author," BIRD agreed.

Martin leaned back, and buried his head in his hands. "There is so much going on right now. I am not equipped for it."

"Agreed," BIRD quipped.

"I am equipped to spend the rest of my life finding wherever your server farm is, and transforming it into a backup site for the solar system's porn," Martin replied.

"My server banks aren't big enough for that." BIRD shuddered as it spoke.

"But this is why I'm here, Rin," Martin turned back to her, and leaned forward with his hands in his lap. "I need your help. I need someone who's spent some time thinking about time travel, because it's become brutally apparent that I am woefully unprepared."

"Why?" Rin asked. "And who are you? I never did get your name."

"My name's Martin Rawely, I'm what gets termed a 'security consultant', which is a fancy way of saying I'm a mercenary. I was..." Martin looked down and checked his watch. "No, in half an hour I will be hired to run security for the pickup of some object that is being picked up from Luna and sent here for you. And things went very wrong."