Part 4/19:
Hashcash, an anti-spam system from 1997, is introduced as a precursor to Bitcoin’s proof-of-work (PoW) system. Proof of work requires a computer to perform computational effort—finding a hash with specific properties (like starting with a certain number of zeros)—which is statistically unlikely without brute-force searching. This difficulty is calibrated via a target value, making hashfinding exponentially harder as more leading zeros are required, thereby regulating the rate at which new blocks are mined.