Billionaire Bill Gates said he thought the NFT was “100% based on a greater fool theory,” a financial concept that even overpriced assets can make money if their owner finds a greater fool to sell.

Speaking at a climate change event hosted by TechCrunch, Gates said he preferred to invest in assets with tangible results, such as a farm or factory, “or a company where they make products,” and that he had no cryptocurrency or NFT assets.

Gates joked that “expensive digital images of monkeys” would “improve the world immensely,” referring to the much-hyped NFT collection Bored Ape Yacht Club.

This is not the first time Gates has expressed skepticism about cryptocurrencies. In an interview in February 2021, he worried about the dangers to ordinary investors buying Bitcoin, especially when the value of cryptocurrency was so volatile and its value could fall due to a tweet from a well-known investor such as Elon Musk.

“I do think people get bought into these manias, who may not have as much money to spare,” said Gates in 2021, “if you have less money than Elon, you should probably watch out.”

Gates’s warnings were not superfluous, as the effects of Covid-19 and Russia’s full-scale invasion of Ukraine accelerated world inflation, causing panic among owners of intangible assets. The world’s largest cryptocurrency Bitcoin is now at its lowest level in 18 months.