Where does the money being distributed come from ?

Earlier today, I was talking with a friend about steemit. I was explaining to him how people get paid for posting content, or replying to articles. And talking about steem and steem dollars (to the best of my knowledge), and about the interest being paid.

Then this question came up: But, where does the money come from ? And I had no idea what to answer.

Can anyone explain this to me.

Thanks