"However, when fiat currency has negative real interest, only a fool wouldn't look elsewhere," Musk said in a tweet. He also defended Tesla's action to invest in bitcoin, saying that the difference with cash made it "adventurous enough" for the S&P 500 company to hold the cryptocurrency. Tesla's $1.5 billion bitcoin purchase set the cryptocurrency soaring toward this week's record peak above $50,000 while Musk's recent promotion of dogecoin on Twitter also lifted the price of that cryptocurrency.