Coconut can be considered a fruit or a nut, although it is most commonly referred to as a drupe. A drupe is a fruit with a tough protective covering, such as peach or olive. All drupes have three layers: the exocarp, the mesocarp and the endocarp.
The exocarp, which is the outermost layer, is fleshy, while the mesocarp, or middle layer, is fibrous. The endocarp directly envelops the seed and is very hard and durable. Coconuts are particularly versatile drupes, as parts of them can be used for food, fuel or as constructive materials. In World War II, coconut water was used intravenously when saline solution was unavailable.