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Blockchain bridges also referred to as cross-chain bridges, are the connection that allows the transfer of tokens or arbitrary data (other data) from one chain to another.
Blockchain bridges can do many things, but token transfer is the most common utility. Since its inception, one of the biggest problems of blockchain technology is the inability to work together. While they are very fluid and somewhat efficient as single entities, each blockchain is limited by the walls of its domain. Often, this can lead to high transaction costs and congestion, as in the case of Ethereum.