This computer science lesson is the third in a series about cryptocurrency and non-fungible tokens, also known as NFTs. The series examines some of the technologies underlying various cryptocurrencies such as Bitcoin, as well as some the ethical, cultural and legal implications of these digital currencies. In this particular video you will meet the blockchain data structure. You will learn that transactions are packed into data blocks and that these are added to the blockchain in a process known as mining. You will also learn about the contents of a block header including, crucially, a double SHA256 hash value, which is derived from the previous block’s header and serves to link blocks together. For security reasons, you will see that mining is deliberately made more difficult in a process known as proof of work. Proof work involves discovering a number used only once, called the nonce, in order to generate a hash value that falls below a particular difficulty target.