Bitcoin and the Problem It Solved

2.1 Why digital money is hard

At the end of the last chapter, Mint was looking up how long a Bitcoin transfer takes. Before we get there, we need to understand why creating digital money without a bank was considered nearly impossible for decades.

Think about a photo on your phone. If you send it to a friend, you still have it. You've made a copy, and copying digital things is free and perfect. That's wonderful for photos. It's a disaster for money.

If digital money could be copied the way a photo can, Mint could send the same $500 to a sibling, then send the exact same $500 to a shop, then to a friend. Each recipient would think they'd been paid. This is called the double-spending problem, and it's the central challenge of digital money.

Physical cash doesn't have this problem. When Mint hands someone a banknote, Mint no longer has it. The physical object can only be in one place.

2.2 How banks solve double spending

Banks solve double spending with a ledger, a record of who owns what. When Mint pays someone digitally, the bank subtracts the amount from Mint's balance and adds it to the recipient's. If Mint tries to spend the same money twice, the bank checks its ledger, sees the balance is already zero, and rejects the second payment.

This works well, but it means everyone has to trust the bank to keep the ledger honestly, keep it secure, stay open, and not block payments it doesn't like. The bank is a trusted third party, and as we saw in Chapter 1, trusted third parties come with fees, delays, and the power to say no.

For years, computer scientists and cryptographers tried to build digital money that didn't need a trusted third party. Several clever attempts came close, but none fully solved double spending without a central authority. Bitcoin did.

2.3 Bitcoin's solution: everyone keeps the ledger

Bitcoin's key insight was simple to describe and hard to build: instead of one bank keeping a private ledger, thousands of computers around the world each keep a full copy of the same public ledger.

"Wait," said Node. "If everyone keeps their own copy, how do they agree? What stops me from just editing my copy to give myself more money?"

"That's exactly the right question," said Hash. "Editing your own copy is easy. Getting everyone else to accept your edit is the hard part, and Bitcoin makes it practically impossible."

Here's how it works at a high level. When Mint sends bitcoin, the transaction is broadcast to the network. Special participants called miners collect pending transactions and bundle them into a block. To add their block to the shared ledger, a miner has to win a computing competition, solving a puzzle that takes enormous amounts of computing power and electricity but is easy for everyone else to check. This system is called proof of work.

Once a miner finds a valid solution, they broadcast the block. Every other computer on the network checks it: are the transactions valid, does each sender actually own the bitcoin they're spending, has anyone tried to spend the same coins twice? If everything checks out, everyone adds the block to their copy of the ledger.

The miner who wins is rewarded with newly created bitcoin plus the transaction fees in that block. This reward is what motivates miners to spend money on hardware and electricity, and it's also how new bitcoin enters circulation.

To cheat, someone would need to redo that expensive work faster than the rest of the network combined. On a network as large as Bitcoin's, that would cost an extraordinary amount of money, and even if it succeeded, it would likely destroy the value of the very bitcoin the cheater was trying to steal. The system is designed so that honesty is the most profitable strategy.

Chapter 2 · Two ways to stop double spending

On the left, Mint sends $500 through a bank, which checks its single ledger and rejects a double spend; you must trust the bank. On the right, Mint sends bitcoin that is broadcast to many identical ledger copies, each of which checks and rejects a double spend, so no single party must be trusted.

We'll look more closely at how blocks link together and why the ledger is so hard to tamper with in Chapter 3.

2.4 Mint sends bitcoin

Back to Mint's question. Suppose Mint's sibling has a Bitcoin wallet, and Mint sends them the equivalent of $500 in bitcoin.

Item	Bank transfer	Bitcoin transfer
Amount	$500	$500 in BTC
Network fee	$15 + ~$10 FX markup	Typically a few dollars or less, varies with demand
Time to first confirmation	3 to 5 days	Usually 10 to 60 minutes
Works on weekends?	No	Yes, 24/7
Needs permission from a bank?	Yes	No

"So it's just better?" Mint asked.

"Not so fast," said Hash. "Your sibling now has bitcoin, not local currency. To pay rent, they'll probably need to sell it on an exchange, which can involve fees and a delay of its own. And the price of bitcoin can move during the time they're holding it. Bitcoin fees also rise when the network is busy. It's a genuinely different tool, with different tradeoffs."

This is an honest pattern you'll see throughout this series. Crypto often removes one kind of friction and introduces another.

2.5 Only 21 million, ever

Remember the properties of good money from Chapter 1? Scarcity was one of them, and it's where Bitcoin is most unusual.

Bitcoin's rules fix the total supply at 21 million bitcoin. No central bank can print more. New bitcoin is created only through mining rewards, and that reward is cut in half roughly every four years, an event known as the halving.

Period	New BTC per block
2009 to 2012	50
2012 to 2016	25
2016 to 2020	12.5
2020 to 2024	6.25
2024 to ~2028	3.125

A new block is added roughly every 10 minutes. As the reward keeps halving, new supply slows to a trickle. More than 20 million bitcoin already exist, the 20 millionth having been mined in March 2026, and the final fractions are expected to be mined around the year 2140. After that, miners will be paid only through transaction fees.

Compare that with fiat currency, where the supply can expand whenever a central bank decides it should. Bitcoin's supply schedule was set at launch and is known to everyone in advance. This predictability is why some people describe Bitcoin as "digital gold."

2.6 You don't need to buy a whole bitcoin

Node frowned. "Isn't one bitcoin worth a fortune? I can't afford that."

This is one of the most common misconceptions. Bitcoin is highly divisible. The smallest unit, one hundred-millionth of a bitcoin (0.00000001 BTC), is called a satoshi or "sat," named after Bitcoin's creator. You can buy $10 worth of bitcoin just as easily as you can buy $10 worth of gold through a fund.

Unit	Bitcoin amount
1 bitcoin	1 BTC
1 million sats	0.01 BTC
1 thousand sats	0.00001 BTC
1 sat	0.00000001 BTC

2.7 From experiment to asset

For its first year or so, bitcoin had no market price at all. It was an experiment shared among cryptographers and hobbyists. One of the most famous early moments came on 22 May 2010, when a programmer paid 10,000 BTC for two pizzas. It was one of the first known purchases of a real-world good with bitcoin. Crypto communities still celebrate the date as "Bitcoin Pizza Day," partly as a joke about how much those pizzas would be worth today.

Since then, Bitcoin has grown from a niche experiment into a globally traded asset held by individuals, companies, funds, and, in some cases, governments. Its price history has been dramatic, with huge rises and painful crashes, which we'll study in Chapter 13.

"So what is it really?" Mint asked. "Money? An investment? Gold?"

"People disagree," said Hash. "Some see it as a currency, some as a store of value, some as a speculative asset. For now, it's enough to understand what it was designed to do: let people hold and send value without a trusted middleman, with a supply nobody can change."

2.8 Try it yourself

Open any Bitcoin block explorer and look at the most recent block. Find three things: how many transactions it contains, the reward the miner received, and the total fees paid. Then look at the time between the last few blocks and see how close they are to the 10-minute average.

As a bonus, use the explorer's search to look up a transaction from the latest block. Notice you can see the amounts and addresses, but not the names of the people involved. We'll discuss what that means for privacy in Chapter 15.

Key takeaways from this chapter

1. Digital things are easy to copy, which creates the double-spending problem for digital money.
2. Banks solve double spending by keeping a private ledger, which requires everyone to trust them.
3. Bitcoin solves it with a public ledger copied across thousands of computers, plus a system called proof of work that makes cheating extremely expensive.
4. Miners bundle transactions into blocks and are rewarded with new bitcoin and fees.
5. Bitcoin's supply is capped at 21 million, with new issuance halving roughly every four years.
6. One bitcoin is divisible into 100 million satoshis, so you can own a small fraction.
7. Bitcoin removes some frictions, like banking hours and middlemen, while introducing others, like price volatility and the need to convert back to local currency.