The Wednesday when the whole time-travel adventure began, I was fiddling with my game controller, trying to make the shoot button more sensitive.

Wednesdays are my intense days. It was a Wednesday back when I took the test for Cooper Tech, where my big sister, Sofia, goes, and a Wednesday when I found out I didn't get in. It was a Wednesday when I didn't get into any of the other schools I was hoping for either and learned I would be going to my current school, the Manhattan Polytechnic Academy. Which means it was also a Wednesday when Sofia stopped calling Poly "Tech for Dummies" and started telling everybody that Poly kids are really very creative.

It's not just bad things that happen to me on Wednesdays, though. I was born on a Wednesday. My family came to America on a Wednesday. And it was a Wednesday both times Jaya Rao and I first met--the Wednesday when I first met her, and the one when she first met me.

I had just figured out how to double the input speed on my game controller. I was messing around with the wires with half my attention, while with the other half I tried to think of a good science fair project. Science fair projects are a big deal in my family. Dad is the chief technology officer at a big media software company downtown, Mom is a cognitive neuroscientist, my brother, Dmitri, is a physics major at MIT, and my sister, Sofia, can't seem to remember she's not actually an immuno-oncologist yet, just a high school junior interning in Franklin-Morse Hospital's immuno-oncology lab.

Me? I'm a student at Tech for Dummies, where the kids are really very creative.

I toyed with the idea of doing something really very creative involving rats. I like rats. They're jumpy and inquisitive, like me. But what, exactly? Something with mazes, or chemicals, or electric shocks? Everything I could think of sounded pretty unpleasant for the rats. Besides, rats have minds of their own. They were sure to make my project skitter off in surprising directions, with unusable results.

That's what usually happens to my experiments, even without rats. I'm great at coming up with clever fixes and mysterious surprises. Unfortunately, science fair judges aren't so crazy about mysterious surprises.

I reconnected the game controller to my computer and launched Gravity Force III. A space raider appeared at the upper left of my screen. I whipped the cursor down to the right, ducking my ship behind a dust cloud. My fix worked! The button moved twice as fast as before, and so did the blaster fire. This was great!

I heard a slither behind me, then a crash. I looked up, startled. A blast of wind had come from nowhere. It had blown my new manga poster off the wall and knocked over my lamp. And--wait! Was something wrong with my eyes? Slowly, right in front of me, an object was appearing.

No, it wasn't my eyes. The thing had heft. It was a machine around the size of a football, made of glittering metal. It had gears and rods and knobs and a little saddle, with two tiny dolls sitting on it. They were moving like they were alive.

Not dolls--people.

But that wasn't even the weirdest part. The weirdest part was that one of the tiny people looked just like me.

"Hi, Leo! Bet you're surprised to see us," said the one who didn't look like me. She was sitting in front of him. The guy who looked like me--exactly like me, with my long face, brown eyes, that stupid curl falling down his forehead--was hugging her tightly around the waist so he wouldn't fall off the saddle.

I should have been too busy with surprise and confusion for anything else, but I felt a distinct jab of jealousy.

That surprised me even more. I never thought much about girls, but when I did, it was the action-graphic type, the kind of girl who wears skintight bodysuits and high-tech, thigh-high boots so she can kick the blaster out of the bad guy's hand while doing a backflip.

The tiny girl on the tiny machine looked nothing like that. She was wearing an old-fashioned dress like something out of an educational video about pioneers. Her knot of black hair had fallen over her left ear, and tufts were sticking out in all directions. Her dress was all muddy. She had soot on her face and a funny chin. She was the most beautiful girl I had ever seen.

"What . . . who . . . where did you come from?" I said. Wow, Leo. Real smooth talking.

"Hi, um, me," said the tiny guy. He was wearing a dorky old-fashioned suit. "It's me, Leo. I'm you. Wow, you're big. Listen, this is important. Read H. G. Wells--"

"What do you mean you're me?"

"I'm you, only later. Well, right now we're the same time, but I was later before. Then I was earlier. But from a linear point of view, I guess I'm always later. But it doesn't matter--"

"What? What are you talking about?"

"It's not important. The important thing is, read The Time Machine."

"I don't understand. How did you get so small?"

"We used a shrink ray," said the girl impatiently, like it was obvious. "Listen, Leo, this is important. When you meet Simon FitzHenry, make sure you stop him from--"

"Jaya! Stop it!" Mini-me put his hand over her mouth. "You'll change history! Ow! Don't bite!"

She pulled his hand away. "I'm trying to change history! Save everybody a whole lot of trouble."

"Cause everybody a whole lot of trouble, you mean. Wow, you're impossible."

"Me? If we just tell him a few things that he's going to know anyway soon, we can stop Simon before he--"

The guy covered her mouth again. "Come on, Jaya. We don't have time to argue about this right now. Ow!"

She spat out his hand again. "Oh, so you're the impatient one now? What do you mean, we don't have time? Time is exactly what we have. We have all the time in the world."

"No, we don't! My sister's coming."

The girl--Jaya--ignored him. "Listen, Leo," she said to me. "You have to tell Simon not to--"

Mini-me leaned around her and pressed a lever. They started to fade, getting softer and more transparent. Jaya was still talking, but I couldn't hear her. The wind sprang up again, knocking my books over. Then they were gone, machine and all.

Not a moment too soon. My door burst open. "Jeez, Leo, what's all the banging?" It was my sister, Sofia.

"Just knocking things over." I picked up lamp and the books and put them back on the desk. I turned my back, hoping she'd go away. I had a lot to think about.

"You know what the trouble with you is?" asked Sofia.

"Yeah. I better by now, because it's your favorite thing to tell me."

"The trouble with you," she said, "is you're growing so fast you don't know where your hands and feet are."

"That's not what you said yesterday. Yesterday the trouble with me was I didn't have the simple human decency to put the milk back in the fridge."

"Maybe the two things are connected," Sofia said, sitting down on my bed. She looked like she was planning to stay awhile.

I tried to make her leave by saying, "Well, I better get back to my project." I didn't think it would work, though.

It didn't. "What project?" she asked, looking pointedly at my computer screen, where my ship was lying in an ignominious heap of fragments. Schist! I'd almost made it to Level VIII before the tiny machine distracted me. That crash was going to poison my score.

I had to admit, it was a little crazy to worry about a game score being destroyed by impossible tiny people riding a science-fiction machine.

"Science fair," I said.

"What's the topic?"

I shrugged. "I was thinking about teleportation or maybe time travel. Maybe I could build like an anti-gravity device. Or a shrink ray."

Sofia waved her hand in the air, the way she does. "There's no such thing."

She was wrong. After what I'd just seen, I knew those things existed. That machine with the little people had to involve teleportation or anti-gravity or time travel. Or maybe all three. It definitely involved a shrink ray. I said, "Sure there is! Physicists can teleport subatomic particles. Just ask Dmitri. Or time travel--you told me yourself you could go back in time if you had a faster-than-light spaceship."

"So you're going to build a faster-than-light spaceship for your science fair project?"

"No, but . . ." Why did Sofia always make everything sound so impossible? "I thought I could work on the theoretical underpinnings. You know, like Dmitri did when he won the Randall Prize."

"Oh, well, listen, Cubby." That's her pet name for me--Leo, lion, cub, get it? She uses it when she's trying to be nice, which means when she's not saying what she's really thinking, which in this case was: Dmitri's a genius, you idiot, and you're . . . not.

See, I can read minds! Maybe I should do my project on telepathy.

"Until you get up to the Randall Prize level," Sofia continued gently, "the judges like to see a nice, clear demonstration of something hands-on. Why don't you try some genetics experiments breeding Arabidopsis?"

"Grow plants?" I knew how that would end: with thirty-two paper cups full of dead dirt.

"All right, Drosophila."

"You want me to breed fruit flies in the apartment? Mom's gonna love that."

"Fine, then. If you don't want my help, why'd you ask me?"

I hadn't, actually, but there was no advantage in pointing that out. "I'm sorry. Maybe Ms. Kang has ideas." Ms. Kang is my science teacher.

"Good plan, Cubby. Let me know what she says." Sofia ruffled my hair, just to rub in how much more mature she was than me, and left me alone with my thoughts.

Now that I had some privacy to think, my thoughts were pretty alarming. What had just happened? Either I'd been visited by a pair of kids straight out of a science-fiction story or I was losing my marbles.

Choice A--the science-fiction story--sounded much better than choice B: wacko Leo. But in my experience, unfortunately, sounding better rarely makes a thing true. That's one reason I never do as well on multiple-choice tests as other people who have the same "natural gifts," as my parents like to call them. I tend to pick the interesting choice.

I knew which possibility Sofia would pick here. There isn't a multiple-choice test known to man that Sofia couldn't ace. She wouldn't hesitate to go for choice B: Leo is loopy.

The truth is, I do sort of see visions sometimes. Sometimes when I'm thinking very hard about a gadget I'm trying to build or fix, I imagine it so clearly it seems real. I see it in front of me, with all its gears and wires. But it isn't actually real, and it certainly never talks. This vision was a whole different kind of freaky.

What if Sofia had seen the tiny people herself? Would she conclude she was crazy too?

Definitely, I decided--and she's so proud of being rational that considering herself crazy would drive her completely out of her mind. It was lucky my visitors had vanished before she came in.

Well, not lucky, exactly. I remembered what the one who told me he was me had said just before they disappeared: "My sister's coming."

That meant the little guy must really have been me! And he'd talked about a time machine and the danger of changing history. That's exactly what I would worry about if I found a time machine: going back to the past and changing something so that my parents never met or messing things up so that World War III started last Wednesday or my family never left Moscow. The little guy on the machine talked just like me.

Except, if he was me, how could he act that way with that amazing girl, Jaya? Calling her impossible! Sitting there calmly on a time machine with his arms around her waist!

Well, Future Me knew her better than I did. Maybe she was impossible. I had no idea what she was really like. I only knew I wanted to find out.

But how could I find her? It's not like I could put an ad on Craigslist: You: Six inches tall, dark complexion, messy hair, gorgeous. We met in my bedroom. You knocked over my lamp. You disappeared before I could get your digits.

Where would I even begin to look for her?

Then it hit me. Maybe I didn't have to! My future self clearly knew her well. Maybe I just had to sit tight and wait until she appeared in my life. It would be pretty soon, too--Future Leo didn't look any older than I am now.

The idea made my insides do a happy little dance. Soon I would know that amazing girl well enough to tell her she was impossible.

Then a less cheerful thought struck me. Jaya and Future Me had a time machine. They were traveling back in time. What if they changed something in the past--or even in their past, my future? What if they snarled up the universe in a way that made me never meet the girl?

Unthinkable. Somehow--somehow!--I would have to stop that from happening.

I wished I had someone to discuss this with. Not my family, obviously. My best friend, Jake, was cool enough not to freak out, no matter what I told him. But he wouldn't be any help. He wasn't interested in thinking too hard about anything.

I would see what my science teacher, Ms. Kang, had to say. She has lots of interesting thoughts about things like whether the universe goes on and on forever or loops back around on itself or what cavemen talked about when they were falling asleep. I wouldn't tell her about the tiny, lamp-knocking-over time travelers, of course, but we could discuss general topics in time travel.

I found her the next day in the little room next to the library, which used to be a coatroom. She was grading tests, bent over in a student desk chair, the kind with a big flat arm for writing on. Ms. Kang gets cold easily, so she's always tugging the sleeves of her sweaters down over her hands. She has very dark, slightly purplish red hair, which is kind of strange--don't most Korean people have black hair? Maybe she dyes it. Her lips are the same color as her hair, but I'm pretty sure that's lipstick.

"Hi, Leo," she said, pushing aside the tests. "What's up?"

"Hi, Ms. Kang. I need to ask you something," I said.

"Okay, shoot."

I suddenly felt self-conscious, so instead of asking about time travel, I said, "Why do you hang out in this little room instead of the science office?"

"I miss being near the library." Ms. Kang used to be the school media specialist before she switched to teaching science. "And nobody knows where to find me here, so I can actually get my work done."

"Oh, I'm sorry! I didn't mean to interrupt you."

I started to leave, but she caught my sleeve. "Not you, silly! Sit down. Is that what you wanted to ask me--why I work here?"

I sat in the other chair. "No, not really. I wanted to ask . . . What do you know about time travel?"

"That's more like it." She rubbed her hands together. "Well, I know that we're all traveling forward in time together, at a rate of one second per second. But that's probably not the kind of time travel you mean. Why do you ask?"

"I was thinking about my science project." That was true, anyway. "Has anyone ever made, you know, a real time machine? Like you could use to go backward and forward in time?"

"Not to my knowledge," said Ms. Kang. "But some physicists think it might be possible. If you could build a faster-than-light spaceship, theoretically you might be able to arrive before you left."

I nodded. "That's what my sister says."

"Or you could try to find a wormhole in the space-time continuum."

"A wormhole! Where would I look?"

"Nobody knows for sure, but I have some books that you could start with. There's a good one by Stephen Hawking. The thing is, nobody knows for sure whether time travel is possible. Like Hawking pointed out, if there really are time machines, why haven't we ever met any time travelers?"

"Yeah, but . . ." Yeah, but I have!! I wanted to say. I met two of them yesterday! And one of them was ME! If I said that, Ms. Kang would think I was crazy. "Maybe this isn't where they want to come," I said. "I mean when they want to come. Or maybe there just aren't that many of them. I've never met any travelers from Iceland, but that doesn't mean the Icelanders don't have airplanes."

"True," said Ms. Kang.

"So do you think I should . . . I don't know, try and make a time machine myself?"

"You mean for the science fair?"

I nodded.

Ms. Kang tilted her head. "No harm in trying. I wouldn't count on getting it done for the fair, though. The deadline's at the end of the semester."

"If it took longer, I could use the time machine to go back in time and show myself how to finish," I said. "I could even make an extra time machine and carry it back in time to my present-day self."

Hey! Was that what I had been doing yesterday?

No, probably not--in fact, I'd seemed to be trying very hard not to tell myself anything about time machines.

Ms. Kang shook her head. "Wouldn't that be cheating? The other kids only get a few weeks to work on their projects."

She was right. Plus, that would be changing the past, and the one thing Future Me seemed completely certain about was that I/he shouldn't change the past.

But wait. If the only reason I wasn't changing the past was that Future Me was dead set against it, then by influencing me to not change the past, Future Me was doing exactly what he didn't want to do: changing the past. So in order to save Future Me from changing the past, did I have to change the past myself?

"You okay there, Leo? Your face is all scrunched up."

"Sorry. I was just trying to think the whole time-travel thing through," I said. It's all right, I told myself. I hadn't needed Future Leo to tell me changing the past was dangerous. I knew that already, all by myself. "So if I can't build a time machine, got any other suggestions?"

"But Leo, you're usually so full of ideas! Remember that time you used mirrors and fiber-optic cables to project the view from the roof into the auditorium? Or when you and Jake tuned the toilets to play chords when they flushed? Why don't you do something like that?"

"For my science fair project? But those things weren't real science! They didn't discover anything new or test any theories. They were just . . . fun." That was one great thing about Poly. It may not be as rigorous as my siblings' schools, but the administration can be surprisingly tolerant. Any other school would kick you out for messing with the plumbing.

"The science fair is supposed to be fun too," Ms. Kang pointed out.

I shook my head. "Not if you come from my family. Science fairs are deadly serious. If I do some silly gag project, my brother and sister'll disown me."

"Wow, that sounds like a lot of pressure," said Ms. Kang. "You're not your brother and sister, you know. You have your own unique talents and interests."

"I know," I said. "That's the problem."

"I can't see it as a problem. But if you really don't want to build one of your fun inventions, have you considered submitting something in the History of Science category?"

"History of Science? Is that even a category?"

She nodded, tugging down her sleeves. "Sure. It's not as popular as some of the more hands-on ones, but it's on the list. You'd look at how some aspect of science or technology developed over time."

"Like, write a library research paper instead of doing an experiment?" I liked that idea. No plants or mice to die on me.

Ms. Kang nodded again. "It could be book research, or you could do some hands-on history. You could look at how scientific tools changed over time and how that affected the science. Like telescopes or clocks. Maybe you could build a model."

"The library has lots of books about science and history. But where would I find a bunch of antique telescopes and clocks?" I asked.
Ms. Kang said, "Have you ever heard of the New-York Circulating Material Repository?"