From Aristotle's Politics.
Thales used his skill in forecasting and predicted that the olive harvest would be exceptionally good the next autumn. Confident in his prediction, he made agreements with local olive-press owners to deposit his money with them to guarantee him exclusive use of their olive presses when the harvest was ready. Thales successfully negotiated low prices because the harvest was in the future and no one knew whether the harvest would be plentiful or pathetic and because the olive-press owners were willing to hedge against the possibility of a poor yield. When the harvest-time came, and a sharp increase in demand for the use of the olive presses outstripped supply, he sold his future use contracts of the olive presses at a rate of his choosing, and made a large quantity of money.