On Sunday, the exchange began offering options to bet on something very different: bitcoins.

Why is it different? With stocks and commodities, there's an underlying asset. For stocks, it's a company and the goods or services they produce. For commodities, it's the actual item, like oil, fruit or copper.

With bitcoin, there's nothing like that. Unlike traditional commodities, there's not a physical asset behind it. And unlike currencies, there isn't a central bank ready to back bitcoin up.

Bitcoins live on computer servers. They are produced by complex algorithms and recorded in a digital ledger.