Each blockchain is its own self-contained ledger — Bitcoin's chain has no native awareness of Ethereum's, and vice versa. A cross-chain bridge is infrastructure built specifically to let assets or data move between otherwise-isolated chains.
A common design locks an asset on the source chain in a smart contract or custodial account, then mints a corresponding "wrapped" representation of that asset on the destination chain. To move back, the wrapped token is burned on the destination chain and the original is unlocked on the source chain.
Other designs use liquidity pools on both chains, letting users swap directly without a lock-and-mint step, relying on the pools rebalancing over time through arbitrage.
Bridges have historically been one of the most exploited categories of infrastructure in crypto — concentrating large amounts of locked value behind a single contract or validator set makes them an attractive target, and several major bridge hacks have resulted in losses in the hundreds of millions of dollars.
Using a bridge means trusting its specific security model, whether that's a smart contract, a federation of validators, or a fully trust-minimized cryptographic proof — understanding which one applies is more important than the promised transfer speed.
