The Winklevoss brothers give advice to Bill Gates.

After the Microsoft (NASDAQ:MSFT) founder said in an interview with CNBC that he would short Bitcoin if he had an easy way to do it, crypto investor Tyler Winklevoss gave him advice on Twitter.

“There is an easy way to trade short Bitcoin”, tweeted Winklevoss, who co-founded cryptocurrency exchange Gemini with his twin brother Cameron.

“You can go short using XBT futures on the CBOE exchange. So – less words, more action.”

Futures contracts allow investors to bet on the future price of a security, commodity, or other financial instrument.

Bitcoin futures trading became available in December 2017 on the CME and CBOE exchanges.

CBOE takes prices from the Gemini exchange to calculate the results of futures trades.

The Winklevoss brothers, who first invested in Bitcoin in 2012, were ranked fifth in Forbes’ ranking of cryptocurrency investors.