To decide if a business idea is good, we generally calculate its return on investment (ROI). Yet in doing so, we make the implicit assumption that our cash in hand will always be sufficient to get us through periods of low revenue. And that is quite risky. Everyday, businesses close shop, not because their long-term growth was slow or their value-proposition weak, but because they faced unexpected cash crunches. Building a cash-flow model is what you needed to assess cash-flow risks and better prepare for them.