What is Bitcoin?
The explanation that finally clicks.

If I email you a photo, now we both have it. Money cannot work that way. If I send you £20, I should not still have that £20.

For decades, computer scientists thought you could not fix this without a central referee. Banks are that referee: they keep one record, and if you try to spend the same £20 twice, the bank says no.

Which drops you straight back into trusting the bank. So the first thing Bitcoin had to crack was digital money that can't be copied, with nobody in the middle. The next five ideas are how it did it.

Imagine a notebook that copies itself for everyone in the class. You can only add a new page once you've solved a tricky puzzle and everyone agrees you got it right. Once a page is written, not even the teacher can rub it out.

That is the blockchain. Instead of one bank updating its private records, thousands of computers worldwide record each transaction in public, in order, a new page roughly every ten minutes.

Here's the clever part, the bit that makes it impossible to fake. Each new page carries a tiny fingerprint of the page before it. So the pages are chained together, which is where "blockchain" comes from.

Change a single line on an old page and its fingerprint changes. That breaks the next page, and the next, all the way down. To rewrite one entry you'd have to redo every page since, on more computers than exist. So once something is written, it stays written.

And you don't have to take my word for it. Every page ever written is open for anyone to read.

This is the bit everyone gets wrong, me included. When I heard "mining" I pictured digital pickaxes. It is nothing like that.

Mining is a lottery. Not "solving complex maths," not pickaxes. A worldwide guessing game, and here is exactly how a round works:

Here is why that matters. For the first time, we have something digital that costs real-world energy to make. An email or a song copies for free. A bitcoin does not. You cannot copy-paste more into existence, and that single fact is what gives it weight.

Surely some clever hacker can just cheat all this? Here's why they can't.

Anyone with a laptop can run a "node": a computer that keeps the whole ledger and checks every rule. When a miner adds a block, all these independent nodes verify it. Try to cheat, say by giving yourself extra coins, and they throw it out.

It's tens of thousands of auditors checking the books at once. No one changes the rules unless almost everyone agrees. That's why the motto is "don't trust, verify."

And there's no boss to lean on either. Satoshi, the creator, vanished and never came back. Every other digital money had a founder or company to pressure, regulate or arrest. Bitcoin has no one in charge to target. That isn't a bug. It's the whole point.

Only 21 million bitcoin will ever exist. That is brand new for humanity. Every money in history kept expanding, even gold, which will roughly double again over the next 60 years.

Here's the engine behind that curve. Every 210,000 blocks, roughly every four years, the reward miners get for a block is cut in half. It started at 50 bitcoin, then 25, then 12.5, and today it's 3.125.

So new coins arrive slower and slower, halving after halving, until around 2140 when the reward rounds to zero and the very last bitcoin is mined. In a world where governments print trillions overnight, this schedule cannot be changed.

"Only 21 million" sounds too few, until you remember each one splits into 100 million pieces. Plenty for the planet, and you buy a coffee with a sliver, the way you wouldn't need a whole gold bar for the groceries.

And it isn't a promise you have to trust. The 21 million is written in the code that every node runs. You can read the exact line yourself.

So where is your Bitcoin actually kept? Everywhere and nowhere. What you own isn't a coin in a vault. You own a private key, usually held as twelve simple words, that proves the coins are yours to move.

When it's truly yours, no bank and no government can freeze it or take it. "Not your keys, not your coins."

In Cyprus in 2013 the state simply took 47.5% of large bank deposits. Your bank balance is an IOU you're allowed to look at. Bitcoin held properly is the actual thing, in your hand.