Balsa lumber is very soft and light, with a coarse, open grain. The density of dry balsa wood ranges from 40–340 kg/m3 (2.5–21.2 lb/cu ft), with a typical density of about 160 kg/m3 (10 lb/cu ft).[4] The wood has a low density because it has large cells that contain water, which are evaporated in a kiln over 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.[5] Because 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 — especially free flight model aircraft, as well as full-sized light wooden aeroplanes, most notably the World War II de Havilland Mosquito.[5] Balsa is used to make wooden crankbaits for fishing. Sticks of balsa can be used to make crude dip-pens for calligraphy and can be useful when one needs a specific nib-width or structure absent immediately available metal nibs.