Chapter 06 · The Bitcoin breakthrough

Bitcoin doesn't move money

8 min

Chap.········

Progress

# block 0001

You probably picture Bitcoin as a digital bank account. A balance that goes up, a balance that goes down, bitcoins traveling from one wallet to another. That picture is wrong. Not approximate: wrong. And as long as you keep it in mind, you won't understand what a bitcoin is, or how a transaction works. Let's fix that.

Congrats for making it this far!
We've covered a lot of ground and put it into practice. Nice work.

But one question is still open: "What exactly is a bitcoin? And what happens when you spend one?"

# block 0002

Bitcoin transactions: a new paradigm

In a bank, the answer is obvious: your account shows a balance, you send money, and the system updates the numbers.

Imagine Nicolas wants to send €1,000 to Ms. Smith. Run a transaction in the traditional banking system:

Bank transaction simulation

Bank ledger state

NicolasBalance€2,500

Ms. SmithBalance€800

The balance is updated in the bank ledger.A bank transaction moves a balance.

# block 0003

Bitcoin doesn't work that way at all.

When you say "I'm going to send 1 bitcoin to a friend" (wish I had friends like that), you might picture a digital object leaving your wallet and arriving in someone else's. Well, not quite.

It's a fundamental shift in perspective.

Before we drop a single piece of jargon, get hands-on. Forget the balance: imagine Nicolas's wallet doesn't hold a number, but coins of fixed values (0.8, 1, 0.5 and 2), each of them indivisible. To pay, you have to pick from these coins and combine them.

Nicolas wants to send 1.3 BTC to Ms. Smith. Your turn: put the payment together.

Build a transaction

1. Select Nicolas's coins

Each coin is indivisible in Nicolas's wallet, it can't be split.

2. Amount to send (BTC)

Outputs

Select coins to spend

# block 0004

You just felt it: you can't cut a coin in half. To reach a total of 1.3 BTC, you assemble whole coins (for example: 0.8 BTC + 1 BTC), and when the amount isn't exact, the surplus comes back to you as change.

These "coins" have a name: a UTXO (Unspent Transaction Output). A Bitcoin transaction is not a wire transfer: it consumes existing coins and creates new ones. Think of a UTXO as a coin sitting in your wallet: indivisible, ready to be spent.

UTXO is on the table. Let's move on to an equally central concept.

# block 0005

Your wallet stores zero bitcoins. Weird, right? More precisely: it doesn't store your bitcoins the way files are stored on your computer. What it actually stores is the private keys that let you spend your UTXOs.

The power of keys

Each UTXO is locked by a cryptographic condition. To spend it, you have to prove you own the matching private key.

This key works like a deed of ownership: whoever holds it can spend the UTXO.

Running a transaction means proving you have the right to spend a UTXO, then creating new outputs.

A Bitcoin transaction is not a money movement: it's a transfer of the right to spend.

Let's take the same transaction again, this time from the inputs-and-outputs angle:

Bitcoin transaction simulation

INPUTS

0.8 BTCLocked by Nicolas

CONSUMED

1 BTCLocked by Nicolas

CONSUMED

NEW OUTPUTS

1.3 BTCMs. Smith can spend

0.4999 BTCNicolas can spend (change)

0.0001 BTCMiner fee

Old rights are destroyed. New rights are created.

The right to spend is transferred, not a balance.A Bitcoin transaction is not a monetary transfer: it's a transfer of the right to spend.

# block 0006

And now, a quick check question. Take a shot.

Quiz

In the Bitcoin model, what does the difference between the sum of inputs and the sum of outputs represent?

# block 0007

The model to keep in mind, in one picture:

The UTXO graph

Inputs (existing UTXOs)

Locked0.8 BTC

Locked1 BTC

Outputs (new UTXOs)

Locked1.3 BTC

To the recipient

Locked0.4999 BTC

Change

Value doesn't rise or fall like a balance: the input coins are destroyed, and new coins (spending rights) are recreated.

But one important piece of the puzzle is still missing: how does the network know you actually have the right to spend those outputs? And how do you prove that ownership without relying on a bank, an account, or a central authority?

The answer sits in another fundamental mechanism of Bitcoin: .
On to the next chapter, you're nearly there!