Blockchains in Plain Language

3.1 The village notebook

Imagine a small village where people trade with each other constantly. Instead of using a bank, they keep track of who owes whom in a notebook. The problem is that whoever holds the notebook has a lot of power. They could make mistakes, change entries, or refuse to record trades they don't like.

So the villagers try something different. Everyone keeps their own identical copy of the notebook. Whenever someone makes a trade, they announce it to the whole village, and everyone writes it down. Every so often, the village agrees that a page is complete, and everyone signs off on it before starting a new one.

Now nobody controls the notebook. If one villager secretly changes their copy, it won't match anyone else's, and the village will ignore it.

That's the core idea of a blockchain. It's a shared record, copied across many computers, where new entries are added in batches and old entries are extremely hard to change.

3.2 Blocks and chains

Each "page" in our notebook is a block. A block contains a batch of transactions, a timestamp, and a few other pieces of information.

The blocks are linked together in order, each one pointing back to the one before it, forming a chain. Hence, blockchain. The first block in Bitcoin's chain is the genesis block you looked up in Chapter 1, and every block since then links back to it in an unbroken line.

What makes this chain special is how the blocks are linked. To understand that, we need one concept: the hash.

3.3 Hashes: digital fingerprints

A hash is like a fingerprint for data. You feed any piece of information, a word, a sentence, an entire block of transactions, into a mathematical function, and it produces a fixed-length string of characters. Bitcoin uses a function called SHA-256.

Hashes have three properties that matter.

The same input always produces the same hash. The tiniest change to the input produces a completely different hash. And you can't work backwards from a hash to figure out the input.

Here are two real SHA-256 hashes. Look at how different they are, even though the inputs differ by a single character.

Input	SHA-256 hash
Mint pays Hash 1 BTC	f19185e9dd5fa8a043465de8c868739b7eb6dca24e067b35aef07fd5b0e75ae0
Mint pays Hash 2 BTC	763cecf4f58f18deb1a7c5e28291a217b87dd3a099721ebc71acd1c259b935b5

Change one number, and the fingerprint changes entirely. There's no pattern you could use to guess one from the other.

3.4 Why the chain is hard to tamper with

Here's where it comes together. Each block contains the hash of the block before it. So block 100 contains the fingerprint of block 99, block 101 contains the fingerprint of block 100, and so on.

Suppose someone wanted to change a transaction in block 99, perhaps to erase a payment they made. The moment they change it, block 99's hash changes. But block 100 contains the old hash of block 99, so now block 100 no longer matches. They'd have to redo block 100, which changes its hash, which breaks block 101, and so on, all the way to the newest block.

On Bitcoin, recalculating each block requires the expensive proof of work we discussed in Chapter 2. So rewriting history means redoing the work for every block after the one you changed, faster than the entire honest network is adding new blocks. On a large network, that's effectively impossible.

"So it's like a stack of glass plates," Mint said. "If you crack one near the bottom, every plate above it shows the crack."

"Exactly," said Hash. "Tampering isn't just hard. It's obvious."

This is why people describe blockchains as immutable. Strictly speaking, it's "practically immutable": the older a block is, and the larger the network, the harder it becomes to change.

Chapter 3 · Why the chain is hard to tamper with

Block 99 has been edited, so its hash changes from the original. Block 100 still stores block 99's old hash as its previous hash, so that link no longer matches. Block 101 is unchanged and still matches block 100.

3.5 Nodes and consensus

The computers that keep a copy of the blockchain and check the rules are called nodes. Anyone can run one. Nodes verify every new block and reject anything that breaks the rules, such as someone spending coins they don't own.

The process by which all these nodes agree on the single correct version of the ledger is called consensus. There are two main approaches you'll hear about.

Proof of work is what Bitcoin uses. Miners compete using computing power, and the chain with the most accumulated work is considered the valid one. It's battle-tested and very secure, but it uses a lot of electricity.

Proof of stake is used by Ethereum and many newer blockchains. Instead of spending electricity, participants called validators lock up, or "stake," the network's own coins as collateral. Validators are chosen to propose and confirm blocks, and if they cheat, part of their stake can be taken away. It uses far less energy, though it comes with its own tradeoffs, which we'll explore in later modules.

	Proof of work	Proof of stake
Who adds blocks	Miners	Validators
What they commit	Computing power and electricity	Staked coins
Penalty for cheating	Wasted energy and hardware costs	Losing staked coins
Example	Bitcoin	Ethereum

3.6 Keys, addresses, and ownership

If there's no bank account, how do you prove you own your coins? With a pair of cryptographic keys.

Think of a row of glass mailboxes in a public building. Anyone can see each mailbox and what's inside. Anyone can drop something in through the slot. But only the person with the right key can open a mailbox and take something out.

Your address is like the mailbox. You can share it freely, and people use it to send you crypto. Your private key is the key that opens it. Whoever has the private key can move the funds. On a blockchain, the network doesn't care who you are. It only checks whether a transaction was signed with the correct private key.

This leads to the most important rule in crypto: if someone gets your private key, they get your money, and there's usually no way to reverse it. Most wallets represent your private key as a "seed phrase," a list of 12 or 24 words. Module 2 is dedicated entirely to keeping those safe.

"No password reset?" asked Node.

"No password reset," said Hash. "No customer support line for the blockchain. That's the price of not having a middleman."

Chapter 3 · Keys, addresses, and ownership

Anyone can send crypto into your public address through the slot at the top of the mailbox. Only the holder of the private key, shown as a key fitting the keyhole, can open the mailbox and move the funds.

3.7 Transparent, but not named

Because every node holds a copy of the ledger, anyone can see every transaction on a public blockchain: amounts, addresses, and timing. What they can't see directly is the real-world identity behind each address.

This is called being pseudonymous. Your address is like a pen name. But if your address is ever linked to your identity, for example by an exchange that knows who you are, your full transaction history can become visible to anyone who looks. We'll return to this in Chapter 15.

3.8 Not everything needs a blockchain

Node had one more challenge. "People keep saying blockchain will fix everything. Supply chains, voting, healthcare. Is that true?"

"Mostly no," said Hash. "Blockchains are slow and expensive compared to a normal database. They're worth it when you need a shared record that many parties can trust without trusting each other, and where nobody should be able to change history. Money fits that description very well. Plenty of other things don't."

That's a useful filter to carry through this series. When you see a project claiming to use blockchain, ask what problem the shared, tamper-resistant ledger is actually solving.

3.9 Try it yourself

Search online for a free SHA-256 hash generator. Type your own name and note the hash. Then add a single full stop at the end and hash it again. Compare the two results. You've just seen why changing one transaction would break the entire chain.

Then go back to a block explorer, open any recent Bitcoin block, and find the field called "previous block hash." Click it, and you'll land on the block before. You're walking backward along the chain.

Key takeaways from this chapter

1. A blockchain is a shared ledger, copied across many computers, where transactions are grouped into blocks.
2. A hash is a digital fingerprint: the same input always gives the same output, and the smallest change produces a completely different result.
3. Each block contains the hash of the previous block, so tampering with any block breaks every block after it.
4. Nodes verify the rules, and consensus mechanisms like proof of work and proof of stake decide which version of the ledger is valid.
5. Ownership is proven with private keys. Whoever controls the private key controls the funds, and mistakes usually can't be undone.
6. Public blockchains are transparent and pseudonymous, not anonymous.
7. Blockchains are useful when many parties need a shared, tamper-resistant record without a trusted middleman, not for every problem.