Mempool

The mempool — short for “memory pool” — is a kind of waiting room where your transaction sits after you broadcast it to the network but before it has been picked up by a validator and included in a block. When you hit “send” in your crypto wallet, your transaction does not go instantly onto the blockchain. Instead, it is broadcast to the nodes of the network, which each store it temporarily in their own local mempool while they wait for the next opportunity to process it.

Miners and validators choose which transactions to include in each new block, and their primary incentive is to prioritize transactions that offer the highest fees — which is why paying a higher gas fee generally results in faster confirmation. Transactions that offer very low fees can sit in the mempool for hours or even indefinitely. The mempool is also publicly visible on most blockchains, which has an interesting consequence: sophisticated actors called “MEV bots” constantly monitor it looking for profitable opportunities. If a large trade is spotted waiting to be processed, a bot might insert its own transactions ahead of or around it to capture a profit — a practice known as front-running.

Example: Imagine a busy restaurant where all the orders are posted on a public board before the kitchen processes them. The chef picks the most lucrative orders first — perhaps those from tables that tipped generously in advance. Your order sits on the board until it is picked up, and anyone passing by can see what you ordered. That public board is the mempool, the chef is the validator, and the tip is your gas fee.