This is a simplified, interactive blockchain. Each block's hash is cryptographically generated from its data, a unique number called a "nonce", and the hash of the previous block. Try changing the data in any block. Notice how its hash becomes invalid (turns red) and breaks the chain's integrity. To fix it, click "Mine" to find a new nonce that makes the hash valid again (starting with '000'). You'll then need to re-mine all subsequent blocks to restore the chain.