Why would anyone pay $600,000 for a cat meme, downloaded millions of times, and easily accessible to all on the internet? What does it say about our yearning to purchase and possess that one collector was willing to shell out thousands of dollars for a few-second clip of LeBron James slam-dunking? 

NFTs (the abbreviation stands for “non-fungible token”) are unique digital assets bought with cryptocurrency (Bitcoin, Ethereum or Dogecoin, for example). In some cases, such as memes, the images are widely accessible, and the purchase doesn’t halt their circulation and use. Twitter founder Jack Dorsey’s first-ever tweet from 2006—“just setting up my twttr”—is now valued at $2.5 million (1630.6 Ethereum). In creating a tokenized version of the tweet, he gave the buyer digital...