2.2 Merkle Trees & SPV

Imagine a tournament bracket, but instead of teams playing, you're combining fingerprints. You take every transaction in a block, fingerprint pairs of them together, then fingerprint pairs of those results, and so on, until one single fingerprint remains at the top. That top fingerprint is called the Merkle root, and it goes inside the block header.

Why bother? Because it gives you a clever shortcut: to prove that one specific transaction is in a block, you only have to share a short trail of fingerprints up the bracket — not the whole block. Even with thousands of transactions, the proof stays very small.

Lightweight phone wallets use this trick (called Simplified Payment Verification, or SPV). They don't store the whole blockchain — they just check these little proofs to confirm a payment is real.

The tradeoff: SPV wallets trust that the blocks they see actually follow the rules. Full nodes, by contrast, re-check every transaction themselves. SPV is convenient; running your own node is the gold standard.