The double-spend problem

In plain words

Anything digital is easy to copy. What stops someone spending the same coin twice? Before Bitcoin: a bank, keeping the books, asking your trust. Bitcoin's answer: show every payment to everyone, let the network agree on the order, and honor only the first spend. Once your payment is buried under confirmations, spending those coins twice is practically impossible.

Why it matters

• The unsolved problem of digital cash — for decades.
• Solving it without a bank is Bitcoin's core invention.
• Order, once agreed, is final. That is what confirmations protect.

Not to be confused with

• Copying bitcoin — you can copy keys and wallet files, but the network only honors each coin's first spend.

Go deeper

• Proof of work
• Confirmation
• Blockchain