Remember Yap? Everyone shared one memory of who owned each stone. Now you, Ali, Ben, Cara and the whole class each keep a copy of one notebook.
The notebook records every payment. You could secretly change an old page.
Will you try it?
Try to change an old page. Then see what everyone else's copy says.
Nobody can cheat alone.
Many computers keep the same notebook. That is Bitcoin. Each one checks every change for itself, and anyone can look. Nobody owns it.
Money is a battery for your effort. Remember when Ali made lots of new coins? Bitcoin's rules say only 21 million will ever exist. Computers that add pages earn new coins.
As the years pass, do new coins come faster or slower?
Every few years, the reward for a new page is cut in half. Slide through these halvings and watch how many bitcoin exist.
New coins arrive slower and slower.
The reward is cut in half about every four years. By October 2026, more than 20 million were made. The last one is expected around 2140.
Cara moves far away. You send her bitcoin from your phone. No bank has to say yes.
But it isn't final straight away. A new page comes about every 10 minutes, and a payment usually lands on one within an hour. Then it can't be undone.
In 2010, a programmer paid 10,000 bitcoin for two pizzas. It was the first known time bitcoin bought goods.
So one bitcoin bought just a tiny slice of pizza. The price of a bitcoin can rise or fall fast.
Source: Bitcoin (Wikipedia) · Bitcoin price is volatile (bitcoin.org) · Payments take about 10 to 60 minutes (bitcoin.org)
Ben says: "Only 21 million will exist. So the price can only go up!"
Is Ben right?
A fixed amount is not a fixed price.
Nobody can make more than the rules allow. But what people will pay for a coin can go up or down.
Remember how money can leak? See how Bitcoin does on each one.
Bitcoin has fans. It also has real doubts. Let's look at both sides fairly.
What are the honest doubts? Find out in chapter 10.
🧠 What you learned
- Many copies of one notebook stop cheating
- Only 21 million, and new coins slow down
- A fixed amount is not a fixed price
🎯 Question 1 of 10 · Easy
Why was your secret change to the notebook thrown out?
How many bitcoin do the rules allow, in total?
How is Bitcoin's notebook like the stones on Yap?
About how often is the reward for a new page cut in half?
You send bitcoin to Cara far away. Who has to approve it?
You send bitcoin to the wrong person by mistake. What can the system do?
In 2010, 10,000 bitcoin bought two pizzas. What did one bitcoin buy?
In our sort game, which one is still a worry for Bitcoin?
Does a fixed number of coins mean a fixed price?
A friend says: "Bitcoin cannot lose money, because only 21 million will exist." What is a fair reply?
Well done!
You earned a badge: Bitcoin: money nobody can print more of
Education, not financial advice. Never put in money you can't afford to lose.
👩🏫 Grown-up corner · 1 of 7
The notebook has a proper name
Bitcoin's notebook is called the blockchain. Each page, or block, stores a fingerprint (a hash) of the page before it, so changing an old page breaks every page after it. Many computers, called nodes, each keep their own copy and check new blocks against the same rules for themselves. They do not vote. Every payment is stored publicly and permanently. Chapter 11 shows how it stays safe.
👩🏫 Grown-up corner · 2 of 7
Spending twice, and signing
A file can be copied, so digital money could be spent twice. Bitcoin stops that with one shared notebook in one agreed order, where each coin can be spent only once. A payment is signed with a secret key, a mathematical proof that it came from the owner, and the signature stops anyone changing it afterwards. Satoshi Nakamoto shared the idea in a white paper on 31 October 2008, and the network was created on 3 January 2009.
👩🏫 Grown-up corner · 3 of 7
Why "21 million" holds
The reward for adding a block is halved every 210,000 blocks. It was 3.125 bitcoin in 2026, and the last coin is expected around 2140. Every full node checks new blocks against the same rules, and the 21 million limit is part of how Bitcoin is designed. By blockchain.info's count in October 2026, about 20.1 million (95.7%) had been made. Our own sum: at about 144 blocks a day, new coins add under 1% a year. Each bitcoin can be split to 8 decimal places.
👩🏫 Grown-up corner · 4 of 7
Fixed supply is not fixed value
A fixed supply answers one question: can more be made? It does not answer another: will people still want it? Bitcoin.org says the price can unpredictably rise or fall over a short time. It crashed in 2018 and fell again in 2022. The Bank of England, the UK's central bank, aims for 2% inflation, because it worries about falling prices (chapter 5's Grown-up corner explains why). Whether that worry applies to bitcoin is a debate.
👩🏫 Grown-up corner · 5 of 7
Why many economists dislike a fixed money
Some economists object to money tied to a fixed stock. The Federal Reserve's history says the gold standard linked countries' policies, so one country's rate rise triggered recessions around the globe. It says some leaders put defending gold ahead of aiding failing banks, while the US money supply fell by nearly 30 percent from 1930 to 1933. They argue a money that cannot expand cannot be eased in a panic. Supporters reply that a fixed supply protects savers from printing.
👩🏫 Grown-up corner · 6 of 7
Michael Saylor's case: harder than gold
Michael Saylor says there will never be more than 21 million bitcoin, even a thousand years from now. The series argues this is fixed by mathematics and by the many people who keep running the same rules (chapter 11), which no physical thing like gold can promise. He likens gold to a leaking battery: you must guard it, carry it and trust whoever holds it. He is a committed Bitcoin supporter, so this is one side of the debate.
👩🏫 Grown-up corner · 7 of 7
Fixed rules, and fast lanes on top
Saylor argues that Bitcoin's basic settings, such as a new block about every 10 minutes, should never change. He compares them to gravity: change one, and everything built on top falls down. His answer to speed is layers: a slow, solid base for big payments, and fast lanes such as Lightning on top for small ones. bitcoin.org says a first confirmation usually takes 10 to 60 minutes, so a bank app may feel quicker. But quick is not settled: Wikipedia says clearing houses for many small bank payments work in batches, and settlement between banks may be delayed for days.