Millions of dollars are being poured into digital artworks - or "non-fungible tokens" - in a speculative craze that has set the internet ablaze. So-called NFTs are tradeable, collectable digital assets that can take the form of an image, a video, or a piece of music. Some are fetching huge prices, and they're extremely controversial. New York Times technology journalist Erin Griffith spoke to Corin Dann.