Worldwide, companies hold billions of dollars in inventories. The main reason is to create a buffer that balances the differences between the inflow and outflow of goods. Inventories can be thought of as water tanks: there may be a constant inflow of water that is pumped into the tank by a pump, while the outflow is low at night, high in the morning (when people get up, take a shower, etc.), it then decreases significantly until the demand again increases in the evening (when people come home, do laundry, etc.), just to fall off again for the night. Other, popular, examples include grocery stores whose inventories consist of various foodstuffs awaiting sale to its customers. Here, the delivery of the goods is in bulk whenever a delivery truck arrives, while the demand is unknown and erratic. In the case of hospitals, they have in stock medical supplies, bed linen and blood plasma. Again, the demand for these items is uncertain and may differ widely from 1 day to the next.