French children are told a story in which they imagine having a pond with water lily leaves floating on the surface. The lily population doubles in size every day and, if left unchecked, it will smother the pond in 30 days killing all the other living things in the water. Day after day, the plant's growth is small and so it is decided that it shall be cut down when the water lilies cover half of the pond. The children are then asked on what day will half of the pond be covered in water lilies. The solution is simple when one considers that the water lilies must double to completely cover the pond on the 30th day. Therefore, the water lilies will cover half of the pond on the 29th day. There is only one day to save the pond. (From Meadows et al. 1972)
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