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# How Is Blockchain Immutable?
- URL: https://121gigawatts.org/how-is-blockchain-immutable/
- Published: 2020-03-07T00:00:00.000Z
- Updated: 2026-10-27T18:58:04.000Z
- Description: Immutable is just a fancy way of saying a blockchain’s contents cannot be changed. See what happens if Bugs Bunny tries to change the contents of a block.
- Author: Zoltán Marić
- Tags: Blockchain, #Import 2026-10-27 12:17

In [episode 5 of The Building Blocks of Blockchain](https://121gigawatts.org/the-building-blocks-of-blockchain-putting-it-all-together), we explained in detail how the mechanics of blockchain work. We didn’t really show how all of that makes blockchain ***immutable***, though. Because immutability is one of the crucial features that makes blockchain interesting for application in the energy industry, I thought it worth explaining in more detail.

*Immutable* is just a fancy way of saying a blockchain’s contents cannot be changed. Let’s see what happens if Bugs Bunny attempts to change the contents of a block.

![Bugs Bunny in Front of Piano Saying "Watch This"](https://121gigawatts.org/content/images/2026/10/Bugs-Bunny-Eyebrows-1-q75-98163bf7b0_o.webp)

Source: [Giphy](https://giphy.com/gifs/looneytunes-smile-9Jcw7ESF7uk7cQwUjO?ref=121gigawatts.org)

Let’s take the same block we used in the [*“Putting It All Together”*](https://121gigawatts.org/the-building-blocks-of-blockchain-putting-it-all-together) post:

```
1. Daffy Duck receives 12.5 Bitcoins
2. Yosemite Sam gives Wile E. Coyote 4 Bitcoins
3. Porky Pig gives Pepé Le Pew 3 Bitcoins
4. Bugs Bunny gives Marvin the Martian 1 Bitcoin
5. Wile E. Coyote gives Elmer Fudd 2 Bitcoins
6. Elmer Fudd gives Porky Pig 7 Bitcoins
---
previous_block: 00bc30f8fae8f8b3ca113390fb4cfa16fd01084189fe285805a43c22bd81c739
nonce: 117
hash: 00639a07198476c9436cbbe1836ca8ca23c6a4792f8ebcba68732b56e9b81fae

```

And let’s say this is ***block number 5*** in the blockchain.

Bugs’ first idea is to just ***change transaction number 2*** in the block:

```
2. Yosemite Sam gives Wile E. Coyote 4 Bitcoins

```

so that ***he becomes the recipient*** instead:

```
2. Yosemite Sam gives Bugs Bunny 4 Bitcoins

```

He quickly realises this won’t work, because Yosemite Sam’s [digital signature](https://121gigawatts.org/the-building-blocks-of-blockchain-digital-signatures) will not check out any more, and he can’t create a valid signature without knowing Sam’s private key.

He then figures he could just ***remove*** the transaction in which he gave Marvin his Bitcoin, essentially stealing it back. Now block number 5 looks like this:

```
1. Daffy Duck receives 25 Bitcoins
2. Yosemite Sam gives Wile E. Coyote 4 Bitcoins
3. Porky Pig gives Pepé Le Pew 3 Bitcoins
4. Wile E. Coyote gives Elmer Fudd 2 Bitcoins
5. Elmer Fudd gives Porky Pig 7 Bitcoins
---
previous_block: 00bc30f8fae8f8b3ca113390fb4cfa16fd01084189fe285805a43c22bd81c739
nonce: 127
hash: 0091ba2887d8a267c462c8d6f0a538e87188748308e15c644e25e81e479eeaf6

```

However, removing the transaction ***changes the*** [***hash***](https://121gigawatts.org/the-building-blocks-of-blockchain-cryptographic-hashes) of the block:

![](https://121gigawatts.org/content/images/2026/10/Hash-change-on-removed-transaction-lossless-337b4ddd30-small_o.webp)

Bugs Bunny removes a transaction from the block, causing its **hash** to **change**

The above calculation was made using [this online hash calculator](https://keccak-256.cloxy.net/?ref=121gigawatts.org). The new hash is:

```
1f2d89d4cb8ed12233ea4a4c9578b64ff5071dfd2a1213eadf99f2d92e587bae

```

which no longer matches the `hash` declared above. What’s more, it no longer provides a valid solution to the puzzle, as the new hash does not start with 2 zeroes! That means Bugs Bunny needs to solve [the puzzle](https://121gigawatts.org/the-building-blocks-of-blockchain-putting-it-all-together/#where-s-the-puzzle) all over again!

That’s still not enough setback for Bugs Bunny to give up, so he goes about solving the puzzle. It takes him quite a while, but he manages to solve it eventually. He finds that the nonce `398` results in the following hash for his updated block number 5:

```
00be3133fa9810016c39305eb0dffc206ab98871b39fceec1b7d7c35911cb356

```

Now he has a valid-looking block which he could start spreading around. However, while he was solving the puzzle, there were several new blocks added to the blockchain! The next block, number 6, contains the **old hash** of block number 5 in its `previous_block` field. If Bugs Bunny were to update the `previous_block` field in block 6, *its* hash would change, which means he would need to solve a puzzle again, and then again for block 7, and 8, and each subsequent block.

> ***Any change to a block makes the hash of that block and all subsequent blocks invalid.***

*Just call it a day, Bugs.*

![Bugs Bunny Shrugging and Walking Away](https://121gigawatts.org/content/images/2026/10/Bugs-Bunny-Shrug-q75-cac4cd897a_o.webp)

Source: [Giphy](https://giphy.com/gifs/looneytunes-confused-bye-58FDuiRsLPfmpd1U6X?ref=121gigawatts.org)