The second requirement of creating a single truth is having many computers trying to solve the puzzle simultaneously. 

For the computers to solve the puzzle and add a new block, they have to keep a copy of the entire blockchain. These computers are called ‘nodes’.  

That’s where a concept called "decentralization" comes in: Ideally, anyone can download and hold a copy of an entire public blockchain on their computer so that if one computer goes out of service, others still have all the data.

🏃‍♀️ Up next: We’ll explain how people are incentivized to become a node and do all the proof-of-work.