In the 1940s, a computer was a building's worth of machinery — racks of glowing glass tubes, miles of wire, the heat of an oven, and a team of people to keep it alive. The phone in your pocket does the same kind of work hundreds of thousands of times faster, and you carry it without thinking. That shrinking — from a room to a hand — is one of the great stories in all of engineering, and it comes down to one part getting smaller, three times over.
The first switch was the valve — a glass tube the size of a small light bulb that could let electricity through or block it. (It is the same kind of on-or-off switch you met in Computing, when everything was built from 0s and 1s.) Valves worked, but they were huge, hot, and burned out like bulbs. A room-sized computer might hold eighteen thousand of them, and one would fail almost every day.
Then, in 1947, came the transistor — a switch with no glass and no glow, made from a specially treated material called a semiconductor. It did the valve's job, but it was tiny, cool, and reliable. The last step was the cleverest: instead of wiring transistors together by hand, engineers learned to print thousands of them at once onto a single sliver of silicon — the microchip. Once you can print them, you can print more, and smaller. And they did, and they have, every year since.
Valve, transistor, microchip — they are all the same idea doing the same thing: a switch that is on or off. What changed was the size. A single fingernail-sized chip today holds tens of billions of transistors, each flipping billions of times a second. The job never changed. The scale changed everything.
When engineers couldn't find why an early computer kept failing, they discovered a moth caught in a relay and taped it into the logbook, writing "first actual case of bug being found". The word had been used for faults before — but that 1947 moth is why a fault in a machine is still called a bug, and fixing one is called debugging.
Open the box and most of what matters comes down to three parts — and the easiest way to keep them apart is to picture a kitchen. You met two of them already, in Lesson 1, hidden inside Babbage's brass engine: he called them the Mill and the Store.
The CPU does the actual work: every instruction, every calculation, flows through it. This is Babbage's Mill, grown up.
RAM is the desk you spread your work across while you do it — fast, but cleared the moment the power goes off. Babbage's Store.
Storage is where everything is kept when the power is off — your photos, files and apps, waiting patiently in the drawer until you ask for them again.
Here is the same idea as a plan of the machine. The CPU does the work; RAM sits right beside it as the worktop it spreads things out on; storage keeps everything safe underneath; and the ports along the edge are the doorways to the outside world.
The reason a computer feels slow when you open too many tabs is almost always RAM running out of room. The worktop gets crowded, so the machine starts shuffling things back and forth to the slower filing cabinet to make space — and all that shuffling is the wait you feel. More worktop, less shuffling.
Storage — the part that remembers your photos when the power is off — comes in two main kinds, and they could hardly be more different inside. One spins; one sits perfectly still.
The older kind is the hard drive, or HDD. Inside is a shiny metal disc — a platter — that spins thousands of times a minute, while a tiny arm sweeps across it to read and write, magnetising tiny patches of the surface. It works rather like an old record player: a spinning disc and an arm that finds the right spot. Because real parts have to move and line up, it takes a moment, and a knock while it spins can damage it.
The newer kind is the SSD, the solid-state drive. "Solid state" means exactly that — nothing moves. It keeps your data in flash memory — banks of chips that hold their contents even with the power off — so there is no disc to spin and no arm to wait for. That is why a laptop with an SSD wakes the instant you open it, and why a dropped phone doesn't lose its photos: there is nothing inside to knock out of place.
The very first hard drive, IBM's RAMAC of 1956, held about five megabytes — roughly one phone photo — and to do it, it stacked fifty discs each two feet wide. The whole thing was the size of two fridges and so heavy it was delivered by forklift. The chip in your phone holds tens of thousands of times more, and weighs less than a feather.
The ports along the edge of a computer are its doorways — how pictures, files and power get in and out. Most carry the name USB, but the shape and the speed have changed over the years, which is why some cables feel quick and others feel like wading through treacle.
| The port | Shape | Roughly how fast |
|---|---|---|
| USB-2 | the older flat rectangle (one way up) | the slow lane — fine for a keyboard or mouse |
| USB-3 | same flat shape, often blue inside | many times faster — good for moving files |
| USB-C | small, oval, and reversible | newer and faster again, and it carries power too |
The friendliest thing about USB-C is that it is reversible — there is no right way up, so it goes in first time, every time. (The older flat plug famously fits only one way round, and somehow never the way you first try.)
Now, how fast? This is where one small trap catches almost everyone, and it reaches back to something you already know from Computing: the difference between a bit and a byte. Remember the rule — 8 bits make 1 byte. The catch is that connection speeds are measured in bits, while file sizes are measured in bytes:
The little b is for bits. This is how the speed of a connection is written — how fast bits flow down the wire.
The big B is for bytes. This is how a file's size is written — and bytes are eight times bigger than bits.
So a connection of "80 megabits per second" only moves about 10 megabytes per second — you divide by eight to get from bits to bytes. That is why a download can feel slower than the big number on the box promised: the box was counting in the small unit. Same trap, one rule to escape it — divide by eight.
That single USB-C port doing everything — power, files, video to a screen, all down one reversible cable — is so useful that in 2024 the European Union made it the required charging port for new phones and tablets sold there. One small oval plug, written into law, so that one charger can power almost anything.
You met the bottom of this ladder in Computing already: a bit is a single 0 or 1, one switch, and 8 bits make a byte — about one letter of text. From there, the words for bigger piles climb in big jumps. Each step up is roughly ×1000 — a thousand times more than the step below it.
Those ×1000 jumps are the same orders of magnitude you work with in Maths — the reason the words climb so fast is that each one multiplies, rather than adds. Skip three rungs, from a kilobyte to a gigabyte, and you have already multiplied by a million. (If you'd like the careful version of bits and bytes again, it's all waiting in your Computing lesson on data.)
Hold onto this ladder, because the AI you can talk to today was trained on something near the top of it — text and images measured in many terabytes, even petabytes, worked on by rooms full of fast chips. The reason that became possible at all is the same shrinking story you started with. We'll come back to it.
There are rungs above the petabyte, with grand names most people never need: an exabyte, then a zettabyte. It's estimated that all the data humanity now makes in a single year is counted in zettabytes — that's billions of the terabyte drives you might find in a laptop, every twelve months.
Now watch the three parts work together. We'll follow one photo from the moment you take it, and at each step ask the same question: where is it living right now — the CPU, RAM, or storage? Reveal each step and predict the next before you move on.
Tap each card — what everyday thing is each part most like?
"The cloud" sounds like it floats somewhere in the sky, but it's the most down-to-earth thing imaginable: vast warehouses packed with thousands of the very drives and chips from this lesson, humming away in cooled buildings around the world. When a photo backs up "to the cloud", it's really being filed in someone else's enormous filing cabinet, a long way away.
Fresh one. A connection moves 40 megabits every second. About how many megabytes per second is that? (Divide by eight.)
Fresh one. Climbing the same ladder, the step one rung above a gigabyte — about a thousand of them — is the ______. What's its name?
Here is a real test of understanding: explaining a thing clearly, in words that are yours. Imagine a friend asks you what the difference is between a computer's memory (RAM) and its storage (the SSD) — they keep mixing the two up. In a few sentences, set them straight, in plain words of your own. Use a picture or comparison if it helps — the worktop and the filing cabinet, or one of your own. Make it land.
Open with the one-line difference, then make it stick with a picture. A single clear comparison beats a long technical list.
strong You got the heart of it down in your opening line — that one keeps things and one doesn't — which is exactly the distinction your friend was missing. Leading with the clear difference, before any detail, is the move that makes an explanation effortless to follow.
try this The place to sharpen is the comparison. A picture only helps if it does the work the words can't — so make sure yours shows why RAM empties and storage doesn't, rather than only that they differ. One image, carried all the way through, lands harder than two half-drawn ones.
to add A nice final touch would be a single everyday example your friend would recognise — losing unsaved work when a laptop dies, say. Ending on something they've felt themselves makes the idea stick long after they've read it.
A short film from Computerphile about the MegaProcessor — a working CPU built the size of a room, with lights for every switch. Watch for the very thing this lesson described: a processor doing its work, and the memory sitting right beside it.
If you'd like to see how the chips themselves are made, the BBC documentary Secrets of Silicon Valley, and the Royal Institution's filmed Christmas Lectures on computing, both tell that story well — fine for 13 and 15, and good ones to watch with Dad or with Isabel.
You've lifted the lid. You saw the switch shrink from a glowing valve to a transistor to a printed chip — the reason a room became a hand. You met the three jobs: the CPU that does the work (Babbage's Mill), the RAM that holds it while you work (his Store), and the storage that keeps it when the power's off — spinning platter or silent chip. You learned the doorways and the trap of bits and bytes, and you climbed the ladder from a single switch to a petabyte. Next time we ask the question hiding behind all of this: why did these parts grow so powerful, so fast — fast enough to make today's AI possible at all? That story starts with a grain of rice on a chessboard.