Teaching model

How blocks form a chain

Genesis has no parent. Every later block points at the one before it. This page uses IDs and short toy hashes — not full Bitcoin headers.

Read the chain

  1. Start at Genesis — the first block. It has no previous ID.
  2. Follow the arrows. Block 2 stores prev ID 1, block 3 stores prev ID 2, and so on.
  3. Each toy hash is SHA-256 of that block’s id + prev link, shortened to 8 characters.

In a real chain the link is the previous block’s full hash, not just an ID. Change one block and the next hash no longer matches — that is why the chain is tamper-evident. The mining game is where you search for a hash that meets a target.

A six-block teaching chain

Left to right: genesis, then each child. On a phone the chain stacks top to bottom.

  1. Genesis

    Block 1 · start

    ID
    1
    Prev ID
    none
    Toy hash
    93f6574d

  2. Points to 1

    Block 2

    ID
    2
    Prev ID
    1
    Toy hash
    b18747d2

  3. Points to 2

    Block 3

    ID
    3
    Prev ID
    2
    Toy hash
    4a026b65

  4. Points to 3

    Block 4

    ID
    4
    Prev ID
    3
    Toy hash
    caefeb4e

  5. Points to 4

    Block 5

    ID
    5
    Prev ID
    4
    Toy hash
    29affcc9

  6. Points to 5

    Block 6

    ID
    6
    Prev ID
    5
    Toy hash
    f9123aee

Simplified teaching model. These are not live Bitcoin headers and they do not pay coins.

Genesis is special

The first block has no parent to point to. Everything after it hangs off this start. That is why it is called the genesis block.

Why linking matters

If someone edited block 3, its toy hash would change. Block 4 still stores the old prev link, so the chain would no longer line up. Later blocks inherit that break.

Next

This visual sits on the Learn path after the blockchain lesson. The live chain tip is on that page. The mining game is where you change a nonce until a hash meets a target.

Learn hub Blockchain lesson Try the mining game