https://www.cnbc.com/2018/05/08/tyler-winklevoss-tells-bill-gates-how-to-short-bitcoin.html

Early bitcoin investor Tyler Winklevoss tweeted back at Bill Gates to explain how to bet against the cryptocurrency, after the Microsoft co-founder said he would short it if he could.

"Dear @BillGates there is an easy way to short bitcoin," Winklevoss said in a tweet Monday afternoon. "You can short #XBT, the @CBOE Bitcoin (USD) Futures contract, and put your money where your mouth is!"