Need a quick way to earn $10 USD? Open an account at Coinbase, deposit $100 USD in Bitcoin, and Coinbase will add $10 USD in Bitcoin to your account!
Since the value of Bitcoin is volatile, you could immediately withdraw the $110. Or you could leave the $10 in your account. Why? Because Coinbase will also give you a referral link.
If someone opens an account using your referral link and deposits $100 USD in Bitcoin, Coinbase will add $10 USD in Bitcoin to his or her account — and add another $10 USD in Bitcoin to your account as well! And they will do that for every referral you send them.