As cryptocurrency evolves, it’s less about buying things and more about investing. Regulators are trying to figure out how to protect investors as prices have swung from $20,000 to less than $6,000 in just a few months. But no one has yet decided how to classify bitcoin and other digital currencies. That’s because they’re not really currencies. They’re not exactly securities, to be regulated like stocks and bonds. And they’re not exactly commodities, since there’s no physical product. Nathaniel Popper covers cryptocurrencies at the New York Times and wrote a book called “Digital Gold: Bitcoin and the Inside Story of the Misfits and Millionaires Trying to Reinvent Money.” He spoke with Marketplace Tech host Molly Wood about how both regulators and investors are thinking about bitcoin.