The machines and ideas rewriting the world.

A day of quiet computing

How many computers have you used today? Most people say one, maybe two. Then count again. The phone that woke you up is a computer. So is the chip in the lift, the card reader at the shop and the box that runs the traffic signal. When you sent a message, it crossed a network of other computers you will never see. When an app showed you a video, an algorithm chose it. When your phone recognised your face, a model that learned from examples made a guess.

So you use many computers a day. You just do not see them. This course is about those machines and the ideas inside them: how the internet actually moves your messages, how code and algorithms work, what AI is and is not, and how to keep your data and your accounts safe.

Is that too much for one course? No. Every one of those ideas rests on a few simple ones, and you can learn the simple ones in an afternoon. The rest is putting them together.

What does your gut say?

Three quick questions before we begin. There is no score, and nothing leaves this page. At the end of the course you will see your first answers next to what you know by then.

How this course works

Four parts and 26 chapters, then a wrap-up, about 12 hours, at your own pace. Each chapter has:

• a story, something to try and a question to argue about,
• an "In short" box at the top, so you can see the main points first,
• a link into World 101, Learnacy's map of what the world knows, and often a Learnacy Lab to play with,
• a note for your notebook, and
• a five-question check. Pass it with 4 right, as often as you need.

Pass every chapter check and the final round, and you get a certificate of completion with a link anyone can use to verify it.

Two promises. This course never asks for a password or any real personal data, and nothing you type in it leaves your device except your answers to the checks and, if you ask for a certificate, the name on it. And it names no company or app to praise or blame: when a chapter needs one, it says "a messaging app" or "a map app". Every example is made up and says so, or comes from a real source.

Two courses sit next door. Math Magicians teaches binary, ciphers, prime numbers and strong passwords. Managing Media teaches your digital footprint, deepfakes, chatbots that sound sure, and India's data protection law. This course links to them when it needs them, and goes further.

One rule for the whole course: questions are more important than answers. In computing, "how does that actually work?" opens every box. And it is normal not to know. Nobody understands every layer of a phone, not even the people who build them. Each of them knows a few layers well, and trusts the rest.

Explore in World 101

World 101 is Learnacy's map of what the world knows. Open Computing Fundamentals and look around for five minutes. Note one question that catches you: it may become the question you write on at the end of the course.

A photo for a friend

You take a photo and send it to a friend in another city. A moment later, it is on her screen. How did it get there? Did it fly through the air the whole way? No. Did it travel as one photo? Also no.

Here is what happened. Your phone cut the photo into many small pieces. Each piece got a label with the address it was heading for, like an envelope. The pieces left your phone by radio, reached a tower or your home router, and then raced along cables, passing from machine to machine. Some pieces took one road and some took another. At the other end, your friend's phone collected them, put them back in order and showed her the photo.

Nobody planned the whole journey. Each machine on the way decided only one thing: where to send the piece next. And that is the surprising idea at the heart of the internet.

Packets: the internet's envelopes

A packet is a small piece of data with a label on the front. The label says where the packet came from, where it is going, and which piece of the message it is. How big can one packet be? On Ethernet, the most common kind of wired network, a packet can be at most 1,500 bytes long, label included. Some networks allow larger or smaller ones.

Why cut a message into pieces at all? Think of a road. If one huge truck used the road, everyone else would wait behind it. If the load goes in many small vans, the road can carry vans from many people at once, mixed together. That is what the internet does. Your photo, a video call next door and a stranger's email share the same cables, one small packet at a time.

The opposite is the old landline telephone. When you made a call, the network kept a whole line open just for you, from one end to the other, for as long as you talked, even in the silences. That is called circuit switching. The internet uses packet switching: no line is kept for you, and every packet finds its own way.

Routers: machines that know only the next step

A router is a computer whose job is to pass packets on. Your home Wi-Fi box is a small router. Large routers sit in buildings run by internet providers, joined to each other by cables.

Does a router know the whole road to your friend's phone? No. Does it need to? No. It keeps a table that says, for each group of addresses, which neighbour is closer to them. A packet arrives, the router reads the address, sends the packet to that neighbour and forgets it. The next router does the same.

So what happens if a cable is cut? The routers next to it notice, update their tables and send packets another way. The message still gets through, perhaps a little slower. Nobody has to fix the whole network by hand. That is why the internet bends instead of breaking.

Send a message in pieces

A made-up network: your phone, four routers and your friend's phone. Send a made-up message and watch its packets take their own roads. Then cut a cable and send it again.

Why the pieces can arrive out of order

Packets that take different roads can arrive in a different order. Piece 5 might come before piece 2. So each label carries a number, and your friend's phone puts the pieces back in order before it shows anything. A piece that never arrives is asked for again; you will see how in chapter 4.

So, coming back to the question. How does a photo reach a friend in a second? It is cut into labelled packets, each router sends each packet one step closer, and the phone at the other end rebuilds the photo from the pieces.

Faster internet for cities, or some internet for every village first?

Money for building networks is limited. Where should it go first?

A hard case. One budget. A remote school with no connection at all, or a city hospital whose network is too slow to send scans to specialists.

Explore in World 101

The internet is many networks agreeing to carry each other's packets. Open Networks and the Internet in World 101 and follow one link that surprises you.