The previous step introduced the blockchain data structure. It turned out that the blockchain data structure consists of two major components: an ordered chain of block headers and Merkle Patricia Tries containing transaction data. This data structure was invented to securely store transaction data. But what does storing data in a secure fashion mean in this context? Answering this question is the purpose of this step. This step outlines the consequences of changing data in a blockchain and illustrates how its data structure detects changes. Furthermore, this step highlights the power of hash references when storing data in a change-sensitive manner. Finally, this step explains how to add new blocks to the blockchain data structure correctly.