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!"

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A representative for Gates did not immediately respond to a CNBC request for comment.

Earlier on Monday, Gates said on CNBC's "Squawk Box" that "bitcoin and ICOs, I believe completely [they're some] of the crazier, speculative things."

"I agree I would short it if there was an easy way to do it," he said. Gates said someone once gave him some bitcoins for his birthday, but he sold it a few years later.