Integral to Bitcoin is a public transaction ledger and log known as the blockchain, which shows who owns how many bitcoins currently and records the participants in all prior transactions as well. By keeping a record of all transactions, the blockchain prevents double-spending (copying one bitcoin and spending it in multiple different places) because the record shows that once a bitcoin has been spent, the previous owner no longer controls it. The blockchain is maintained not by a central body but by a distributed network of computers that run a program to solve cryptographic puzzles relating to information in the blockchain.Users who devote computing power to maintaining the blockchain this way are called "miners" because they are awarded in bitcoin when they are first to solve such puzzles - mining is how new bitcoins are generated. The mathematical calculations performed by miners' computers serve to verify that each transaction is valid and add the information to the blockchain. As more bitcoins come into circulation, the puzzles involved in mining them become increasingly difficult, and the rewards are halved at regular intervals, until 21 million bitcoins have been created and production stops.[21] As Bitcoin achieves wider recognition and more people compete to mine the coins, competition for the limited number of bitcoins awarded for solving the cryptographic puzzles becomes more steep and more powerful computers are needed in order to compete - a fact which has spawned a technology boom in sales of Bitcoin mining technology.
