A computer takes information in, works on it by following instructions, keeps what it needs and puts the result back out, all with billions of tiny switches that are either on or off. This course takes the box apart one piece at a time, with something to play with in every lesson, and by the end you can work out why a computer is slow and usually fix it.
Coming soon to iPhone, iPad and Android. Part I free, then $2.99 once. No account, no ads.
Every photo, song and word on a computer is stored as switches that are on or off. Here are eight of them. Each one is worth double the one to its right. Flip them until you’ve spelled something.
Make the letter H (72)
Adds up to 0
Not a letter yet
Talk Card · Lesson 2.4
Every letter is stored as a number. 72 is H and 73 is I. The computer only ever sees numbers, and a chart everyone agreed on.
Usually not. Most old computers feel slow because their memory fills up and they’re still running on a spinning hard drive, and both are cheap to fix. The bigger question is updates: Microsoft ended regular support for Windows 10 on October 14, 2025, and free security updates for home users only run through October 12, 2027.
If your computer can’t run Windows 11, you have four honest options. The course spends its whole running story on the third one, because almost nobody knows it exists.
Enroll in Microsoft’s extended security updates and keep going until October 2027. Then decide.
Sometimes right, especially if several parts are failing. Diagnose first so you know.
A beginner-friendly Linux or Google’s free ChromeOS Flex gets updates, runs fast on old hardware and handles the browser, video calls and email fine.
Back up your files, wipe the drive, and take it somewhere that recycles electronics. Never just the trash.
Dates as of October 2026. Microsoft has moved the extension once already, so check its page before you decide.
Read the full Fix-It Card for a slow computerMost slow computers aren’t short on processor speed. They’re waiting on something slower: memory that’s full, a hard drive with a spinning plate, or the internet. The processor is so fast that every trip to anything else is a long wait by comparison. Here’s how long, if one processor tick took one second.
If one processor tick took one second, and everything else stretched to match.
Talk Card · Lesson 8.3
If a processor tick were a second, RAM is six minutes away and a spinning hard drive is six months. That gap is why an SSD makes old computers fast.
Every message, photo and video gets chopped into small numbered pieces called packets. Each packet travels on its own, passed from router to router toward an address, and nobody plans the whole route. At the other end, the numbers put them back in order. Try it, then knock out a router and watch them go around it.
Talk Card · Lesson 21.1
The internet splits every message into numbered packets that travel separately and get put back together at the end. Break a path and they route around it.
They’re three families of operating system, the boss program that runs everything else. macOS and Linux both descend from Unix, built at Bell Labs in 1969; Windows grew on its own branch. The right one depends on your apps, your hardware and how much you like to tinker, not on which one is best.
Runs on computers from hundreds of makers, at every price, and runs the most games. More choice, more setup, more junk to watch for.
Best forGamers, and anyone whose work software only runs on Windows.
Apple builds the hardware and software together, so it fits, lasts and works smoothly with an iPhone. Costs more, and only runs on Apple’s computers.
Best foriPhone families and creative work.
Free, open source and fast on old hardware, with beginner-friendly versions. Some popular apps and games aren’t made for it.
Best forTinkerers, and old laptops that can’t run the latest Windows.
The browser is the computer. Simple, quick and very hard to mess up. Google’s free ChromeOS Flex brings it to old PCs.
Best forSchoolwork, email and anything that lives on the web.
Nobody invented the computer alone. Every module carries one true story from the history, and these are six of them.
Ada Lovelace · London
Charles Babbage designed a steam-powered calculating engine he never finished. Lovelace wrote out step-by-step instructions for it anyway, and noted it could only do what we know how to order it to do.
Module 1
Kay McNulty, Jean Jennings, Betty Snyder, Marlyn Wescoff, Frances Bilas and Ruth Lichterman · Philadelphia
ENIAC filled a room and had no keyboard. Six women programmed it by plugging cables and setting switches. At its 1946 unveiling, they weren’t introduced.
Module 3
Federico Faggin, Ted Hoff and Masatoshi Shima · Intel
Built for a calculator, the Intel 4004 had about 2,300 transistors. Today’s chips have billions and still run the same fetch, decode, execute loop.
Module 6
Grace Hopper · U.S. Navy and Remington Rand
People told her computers could only do arithmetic. She built one of the first compilers anyway, which is why almost nobody writes in ones and zeros today.
Module 17
Alex Krizhevsky, Ilya Sutskever and Geoffrey Hinton · University of Toronto
Graphics cards were built for games. Trained on two of them, a neural network crushed a famous image contest, and AI researchers have bought GPUs ever since.
Module 12
Tim Berners-Lee · CERN
He invented pages linked together that any computer could read, built the first browser and server, and CERN put it all in the public domain. The first website is still online.
Module 22
Each module answers one question people actually search for, in four five-minute lessons. Part I is free.
Part I · The hand-me-down
What a computer actually does, why it only speaks in ones and zeros, and how it went from a room to your pocket.
Part II · The brain that isn’t
Switches that make decisions, switches that add, and the loop a processor runs billions of times a second.
Part III · The desk and the filing cabinet
Why your computer forgets when the power goes, why your photos don’t, and the speed ladder in between.
Part IV · It won’t start
The graphics chip, the screen, the board everything plugs into, and the first few seconds after you press the button.
Part V · Mac, Windows or Linux
The boss program that runs everything, the apps on top of it, where your files actually live, and the three families of operating system.
Part VI · Getting it online
How two computers talk, how a message crosses the world in pieces, and why the web isn’t the same thing as the internet.
Part VII · The hand-me-down goes home
The scams that trick people instead of computers, the habits that stop most of them, and how to fix a slow computer yourself.
Scene
The crew hits a problem with the laptop.
Play
You poke at it before it has a name: switches, a pretend processor, a command line.
Name it
Now it gets its real name.
Check
One quick question. Miss it and we slow down.
Back to it
Back to the laptop, and what the idea means for it.
Say it
One sentence you could say to anyone. That’s your Talk Card.
A one-page checklist for the day this happens at your house: the computer is slow, the Wi-Fi works but the internet doesn’t, an email looks like a scam. Print them, screenshot them, stick one on the fridge.
One question to ask whoever’s across the table. You check it off yourself. The app never records anything.
Module 21
Ask someone at home how they think a message gets from here to another country. Then tell them about the cables on the ocean floor.
Grandma Ruth gets an iPad and hands her old laptop to Sarah. It takes four minutes to start, and Mark says it’s junk. Over 24 modules, Sarah, Mark, Jenny and Rob open it up, find out which part is slow and why, fix it for very little money, and put a free operating system on it. Every part of the computer gets met because the crew needs it to fix this one machine.
Every module ends with one. These are the three people need most.
Fix-It Card · Module 24
Restart, read the meters, fix the pinned one, trim the startup list. Then decide. Not before.
Fix-It Card · Module 1
Five minutes and two screenshots, and you know more about the machine than the store did.
Fix-It Card · Module 21
Full bars only mean you’re talking to the router. Work outward: device, router, modem, provider.
The questions people search most, each answered on its own page with the lesson that teaches it.
One box connects the house, the other shares it. Which to restart first.
Your computer’s working space, why it forgets, and how much you need.
Flash chips vs. a spinning plate, and why the swap makes old computers fast.
A few brilliant cores vs. thousands of simple ones, and why AI wanted the second.
Three families plus ChromeOS, where they came from, and how to choose.
Joel Pettigrew
Product leader · Data, analytics and AI
I’ve spent twenty years as a product manager and product leader, building software and the teams that ship it. This is the course I’d hand a sixteen-year-old, or a grandparent, who wants to know what’s actually inside the box and how to fix it when it’s slow. It’s a Pettigrew Lab project, so it’s a family one, same as Data to AI and Product Management.
Read my resumeThe modem connects your house to your internet provider: it’s the box with the cable from the wall. The router shares that connection with every device in the house and makes the Wi-Fi. When the internet goes out, restart the modem first, wait for its lights, then the router. Module 20 covers it.
RAM is the computer’s working space: whatever you have open right now, loaded where the processor can reach it fast. It forgets everything when the power goes off, which is why you save files to the drive. When RAM fills up, the computer borrows the slow drive and everything crawls. Module 9 covers it.
Usually because it’s waiting: memory is full, a spinning hard drive can’t keep up, too many programs start themselves, or something is downloading. Open Task Manager or Activity Monitor and look for the meter stuck near 100%. That part is the bottleneck, and fixing it is often cheap. Module 24 walks through it.
For running a computer, an SSD, by a lot. It has no moving parts, so it finds files almost instantly, while a hard drive has to swing an arm and wait for a spinning plate. Hard drives are still cheaper per gigabyte, so they’re fine for backups and big video libraries. Module 11 covers it.
A CPU has a few powerful cores that can handle any task, one complicated step after another. A GPU has thousands of simple cores that all do the same calculation at once. That’s perfect for drawing millions of pixels, and it turned out to be perfect for training AI too. Module 12 covers it.
Start with your apps. Windows runs the most games and works on computers at every price. A Mac fits best with an iPhone and lasts a long time. Linux is free and makes old computers fast again. None is best for everyone. Pick the one that runs what you need. Module 19 covers it.
DNS is the internet’s phone book. Computers find each other by number, called an IP address, and DNS turns names you can remember, like pettigrewlab.com, into those numbers. No single computer holds the whole list; a chain of servers each knows one level. It usually takes a fraction of a second. Module 21 covers it.
No single person. The internet grew from ARPANET, a U.S. research network first linked in 1969, and Vint Cerf and Bob Kahn designed TCP/IP, the rules that let separate networks join into one. The web, the pages and links you browse, is different: Tim Berners-Lee invented it in 1989. Module 22 covers it.
Phishing is a scam that pretends to be someone you trust, like a bank, a school or a delivery company, to get your password, a code or money. It works on people, not computers. Watch for urgency, odd sender addresses and links that go somewhere else, and go to the real site yourself. Module 23 covers it.
Not forever. Regular support ended October 14, 2025, and free security updates for home users run only through October 12, 2027 if you enroll. After that, it keeps working but gets riskier online every month. You can upgrade, switch to Linux or ChromeOS Flex, or replace it. Module 24 covers the choice.
High schoolers first, written so a parent or a grandparent can take the same lesson. You don’t need to know anything about computers to start. If you can flip a light switch and add two small numbers, Module 1 will make sense.
The app is free to download and Part I is free inside it, along with the slow-computer lesson. Everything else is a one-time $2.99 unlock, the same as Data to AI and Product Management. No subscription, no ads, no account. The three toys on this page are free right now.
It’s coming to iPhone, iPad and Android. Until then, everything on this page works in your browser, including the switches, the speed ladder and the packets.
Start with the Fix-It Card. It’s the same checklist the crew uses on Grandma Ruth’s laptop.
Open the Fix-It CardMake a letter out of eight switches. It’s Lesson 2.4, and it takes a minute.
Try the switchesData to AI is our sibling course: how AI works, starting with the data on your phone.
See Data to AIThe course is written fresh, but these are the books whose ideas it’s built on. Every one is worth reading.
Charles Petzold
Starts with flashlights and Morse code and builds, one step at a time, to a working computer. The spine of Parts I and II.
Find the bookJ. Clark Scott
The plainest walk from a single switch to a working processor, for anyone who can turn on a light.
Find the bookNoam Nisan and Shimon Schocken
Build a computer from nothing but NAND gates, then an operating system on top. The idea behind Module 4.
Find the bookRon White
The classic illustrated tour of every part, from boot-up to the web. Generations of people learned the hardware here.
Find the bookMike Meyers
What IT technicians actually learn: parts, operating systems, networks, troubleshooting. Behind most of the Fix-It Cards.
Find the bookSam Grubb
A friendly guide to how attacks happen and how to stop them, for total beginners. Behind Module 23.
Find the book