Say I'm a farmer and I need X gallons of water next year. How much does that cost me? It costs YX where Y is the cost of water per gallon next year - which is

Say I'm a farmer and I need X gallons of water next year. How much does that cost me? It costs YX where Y is the cost of water per gallon next year - which is unknown. "Unknown" is something of a problem when preparing the budget for next year. I could buy X gallons now, but then I'd need a way to store it. Or, I could agree to pay a specific price now for water that would be delivered in the future.