The Double-Spend Problem

With a physical coin, you can't spend it twice — once you hand it over, it's gone. But digital files can be copied infinitely. So how do you stop someone from sending the same digital coin to two people?

Before Bitcoin, you needed a bank in the middle to keep track. Satoshi's breakthrough was replacing the bank with a network of thousands of computers that all verify every transaction.

When you send Bitcoin, the entire network checks that you haven't already spent it. Miners bundle valid transactions into a block, and once confirmed, it's written in stone. No double spending, no cheating — and no bank needed.