Ah, UTXOs, the building blocks of Bitcoin transactions. It's fascinating how every full-node client keeps track of them in the UTXO set. It's like a virtual game of hide and seek, but with money. "Can you find me in the UTXO set? Nope, I've been spent!" So remember folks, when you spend Bitcoin, you're not just buying something cool; you're also playing a game of UTXO tag!