Crypto With Block

Lessons / How Blockchain Works

Blocks and Chains: Why You Can't Change the Past

Learn what's inside a block, how each block links to the one before it, and why confirmations matter when you send or receive crypto.

9 min read Free lesson Fact-checked Nov 2026

Why you can't change the past

Hi, I'm Block. Imagine trying to secretly change one line in a notebook, when thousands of people hold copies, and every page is stamped with the fingerprint of the page before it. In this lesson: what's inside a block, how each block links to the last one, and why that makes the past so hard to change.

From signature to block

Quick recap. In the last lesson, your wallet signed a transaction and sent it out to the network. But a signed transaction floating around isn't settled yet. Until then, it's just a request. It needs a place in the permanent record that everyone shares. That place is a block.

Think of a block as one page in a shared ledger. Each page holds a batch of transactions, and at a steady pace, a new page is added to the end of the book. Pages only ever get added to the end. Old pages are never torn out or rewritten.

Pages get added. Never torn out.

How steady is that pace? On Bitcoin, the network aims for one new block about every ten minutes. Other chains move faster. On Ethereum, there's a slot for a new block every twelve seconds. Different speeds, same idea: a steady drumbeat of new pages.

New pages, on a schedule.

Inside a block

So what's actually on each page? A block has two parts. The body, which is the list of transactions. And the header, a small summary that sits on top. The body holds the information. The header is where the protection comes from, so let's open it up.

On Bitcoin, the header is tiny, just eighty bytes. It holds a version number, a timestamp, a fingerprint of every transaction in the block, a couple of numbers used for mining, and one more thing. The fingerprint of the block that came before it.

Eighty bytes that point to the previous block.

Let's start with the transactions. Every transaction in the block is hashed, then paired up and hashed again, over and over, until there's one short fingerprint left. That fingerprint goes in the header. Change a single transaction, by even one digit, and the fingerprint changes completely.

Change one transaction, the fingerprint changes.

It's the same trick from the hashing lesson. A hash is a tiny fingerprint that changes if anything in the original changes. So the header doesn't need to carry every transaction to protect them. It just carries their fingerprint, and that's enough to catch any edit.

The fingerprint is enough to catch any edit.

That's the chain

Now the most important field. Every block header carries the fingerprint of the block before it. That's the chain in blockchain. There's no physical chain, just each block pointing back at its parent, all the way down.

And it really does go all the way down. Bitcoin's very first block, called the genesis block, was created on January third, two thousand nine. It has no parent, so where the previous fingerprint would go, it's all zeros. Every Bitcoin block since then links back to it.

Every block links back to block zero.

Its creator even tucked a message inside it: a British newspaper headline from that same day, about a second bank bailout. It works like a date stamp you can't fake. That block couldn't have been made before that headline existed.

Block zero carries a date stamp you can't fake.

Now let's play attacker. Say someone wants to change an old transaction, to pretend a payment they made never happened. They open up block one hundred and edit it. The instant they do, block one hundred's fingerprint changes. And that's where their problem starts.

Change a block, change its fingerprint.

Block one hundred and one stored the old fingerprint. Now it points at a block that no longer exists. The link is broken. To hide the edit, they'd have to rebuild block one hundred and one, then one hundred and two, and every single block after it.

One edit means rebuilding everything after it.

Picture a long chain where each link is forged around the one before it. Swap out one link in the middle, and every link after it has to be broken open and forged again. Changing one block means changing everything that came after it.

Touch one link, and the rest break.

Blocks cost work

And here's why rebuilding is so hard. Making a block isn't free. On Bitcoin, every block takes an enormous amount of computing work to produce, and we'll see exactly how in the lesson on consensus. So rewriting history means redoing all that work, block after block.

Rewriting history means redoing the work.

And while the attacker rebuilds, the honest network keeps adding new blocks to the real chain. They aren't just catching up. They're chasing a target that keeps moving away. Bitcoin's design paper makes exactly this point: as long as honest computers do most of the work, the attacker falls further behind.

The honest chain keeps moving away.

There's a second wall, and it's even simpler. Thousands of computers around the world keep their own copy of the chain. When someone shows up with an edited history, they check every link and every fingerprint. A broken link is rejected on sight. We'll meet those computers properly in the next lesson.

A broken link is rejected on sight.

What a confirmation is

This brings us to a word you'll see in every wallet and on every exchange: confirmations. Each new block stacked on top of the block with your transaction counts as one more confirmation. One confirmation means it's in a block. Six means five more blocks are piled on top.

On Bitcoin, six confirmations became the long-standing rule of thumb for treating a payment as settled. Each extra block makes a reversal dramatically less likely, because an attacker would have to redo more and more work. That's why exchanges make you wait before a deposit counts.

Every block on top makes it harder to undo.

So when you send or receive crypto, there are three stages. Pending means it's still waiting to get into a block, and it can still fail. Confirmed means it's in one. Settled means enough blocks are stacked on top that reversing it isn't practical. Pending isn't paid.

Pending isn't paid.

A screenshot isn't payment

And that gap is exactly where a common trick lives. Someone buying something from you sends a screenshot showing the payment as sent, and wants the item right now. A screenshot is not a confirmation. Neither is a pending transaction.

So check it yourself. Open your own wallet, or a block explorer you've bookmarked, look up the transaction, and see how many confirmations it has. Don't use a link the other person sends you. Don't hand anything over until it's confirmed on the chain, not on someone's screen.

Confirmed on the chain, not on their screen.

Has anyone done it?

But wait. If the past is so hard to change, has anyone ever done it? Yes, on smaller networks. Remember, rewriting history takes more computing power than the honest network. On a big network, that's enormous. On a small one, it can be within an attacker's reach.

In August twenty twenty, a smaller proof-of-work chain, Ethereum Classic, was attacked twice in under a week. In one of those attacks, more than four thousand blocks were rewritten. People call this a fifty-one percent attack: whoever controls most of the computing power can rewrite recent history.

Control most of the power, rewrite recent history.

Here's how a double-spend like that works. The attacker deposits coins at an exchange on the public chain, trades them, and withdraws. Meanwhile, they secretly build a longer chain where that deposit never happened. Then they release it. The network follows the longer chain, and the deposit vanishes.

The longer chain wins, even a dishonest one.

In the first of those attacks, about eight hundred thousand coins were spent twice, worth millions of dollars at the time. Exchanges and a mining pool paused deposits and withdrawals for that coin. The rewrite didn't touch old history. It reached back far enough to hurt anyone who trusted a recent deposit.

Recent blocks are the weakest.

After those attacks, one large exchange raised the waiting time for deposits on that chain to about two weeks. That's why waiting times differ from coin to coin. The wait isn't red tape. It's the exchange making sure the history under your deposit won't be rewritten.

Waiting is protection, not red tape.

Permanent cuts both ways

So what does all this mean for you? On a large, established chain, once a transaction is buried under enough blocks, it's effectively permanent. And that cuts both ways. Nobody can quietly erase your payment. And nobody can erase your mistake, either.

No one can erase it. Including you.

So build habits that respect that permanence. Send a small test amount to any new address first. Check the whole address, not just the first and last few characters. And wait for confirmations before treating money as received. On a blockchain, slow and careful is the only undo button you get.

Slow and careful is your only undo button.

Blocks and chains

Here's the whole lesson in four lines. A block is a page of transactions with a small header on top. Each header holds the fingerprint of the block before it. Change any old block, and every link after it breaks. And confirmations tell you how deeply your transaction is buried. The past is protected by everything built on top of it.

The past is protected by everything on top of it.

Sources

Education only. Not financial advice.

Education only. Not financial, investment, tax or legal advice. Block, our on-screen presenter, is AI-generated, voice included; the research and scripts are ours.