The Bitcoin whitepaper starts with a practical problem: how can two people pay each other online without a central operator deciding which payment counts? Its answer combines signatures, a shared transaction history and proof-of-work. The important idea is not that digital money cannot be copied. It is that conflicting payments cannot both become accepted spending in the same valid history.
Alice controls an unspent output
↓
Payment A → Bob
Spends that output
Spends that output
Payment B → Carol
Spends the same output
Spends the same output
Both may have valid signatures. A valid chain cannot accept both spends.
Original teaching diagram; simplified, not a real transaction.Next: Follow a transaction from Send to confirmation to see how signatures, miners and validating nodes work together.
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