Balsa lumber is very soft and light, with a coarse, open grain. The density of dry balsa wood ranges from 40–340 kg/m³ (2.5–21 lb/ft³), with a typical density of about 160 kg/m³ (10 lb/ft³). The light weight of the wood derives from the fact that the tree has large cells that contain water. After the water is driven off in an extended drying process (kiln dried for two weeks), the large surface area of the resulting holes gives strength. Unlike dry rotted wood, the surface is made of the usual strong cellulose/lignin mix. As it is low-density but high in strength, balsa is a very popular material to use when making light, stiff structures in model bridge tests, model buildings, and for the construction of model aircraft and boats as well as full-sized light wooden aeroplanes.