You give me 0.0005 BTC now and, depending on the interest you choose, you'll receive between 0.0005 BTC and 0.0010 BTC in between 28 and 47 days. The maximum interest I accept to pay is 0.0005 BTC. If I am able to pay you back before, I'll do it, but the interest will remain the same (so you don't loose money if I pay you earlier). Since it's you who pay me at the beginning, I'll simply pick the best offer.