Transactions and the mempool

What actually happens when you hit Send

When you send Bitcoin or Ethereum, you are not moving a coin from one place to another. There is no coin. What you are doing is signing a message — a short note that says 'I, the owner of address A, authorize this much value to go to address B, and I am willing to pay this fee for the privilege.' Your wallet uses your private key to sign it, then broadcasts that signed message to the network. That's the whole transaction. It's a piece of math, not a coin in motion.

After you hit Send, your signed message goes into the mempool — short for 'memory pool.' Think of it like a public waiting room for transactions. Every node on the network keeps its own copy of the mempool. Anyone can look inside and see what's pending. That's important: your transaction is visible to the whole world before it ever gets confirmed. We'll come back to why that matters when we talk about MEV later in this chapter.

How does a transaction leave the mempool? A miner (on Bitcoin) or validator (on Ethereum and most modern chains) picks it up. They build a block by selecting pending transactions, and they almost always pick the ones offering the highest fee first. That's why during congestion your transaction can sit for minutes or hours — the network is sorted by who is willing to pay most. Pay more, jump the line. Pay less, wait your turn.

Once a validator includes your transaction in a block and that block is added to the chain, you get a confirmation. On Ethereum that takes about 12 seconds for the first confirmation. On Bitcoin, about 10 minutes. The deeper your transaction gets buried under newer blocks, the safer it is from being reversed. Exchanges often wait for several confirmations (sometimes 3 to 6) before they credit a deposit, which is why your funds can show as 'pending' on a centralized platform for half an hour or more even after the transaction itself has confirmed on-chain. That waiting period isn't bureaucracy — it's the exchange ensuring the transaction is unlikely to be reorganized out of the chain.

Recap: a transaction is a signed message. It waits in the public mempool. Validators pick the highest-fee transactions and write them into the next block. Your fee is your line-skipper.