Proof of Work secures Bitcoin by requiring miners to expend real computational effort — and therefore energy — to propose new blocks. Because finding a valid block is hard but verifying it is easy, the network can trust any block that meets the difficulty target. This makes attacking the chain economically irrational: an attacker would need to out-spend the entire honest network. PoW's energy use is often debated, but it is precisely this cost that underpins Bitcoin's unrivalled security.