A non-fungible token (NFT) is a non-interchangeable unit of data stored on a blockchain, a form of digital ledger, that can be sold and traded. Types of NFT data units may be associated with digital files such as photos, videos, and audio. Because each token is uniquely identifiable, NFTs differ from blockchain cryptocurrencies, such as Bitcoin. NFT ledgers claim to provide a public certificate of authenticity or proof of ownership, but the legal rights conveyed by an NFT can be uncertain. NFTs do not restrict the sharing or copying of the underlying digital files, do not necessarily convey the copyright of the digital files, and do not prevent the creation of NFTs with identical associated files. NFTs have been used as a speculative asset, and they have drawn increasing criticism for the energy cost and carbon footprint associated with validating blockchain transactions as well as their frequent use in art scams and claimed structure of the NFT market to be a Ponzi scheme.