The wood of the living tree has large cells that are filled with water. This gives the wood a spongy texture. It also makes the wood of the living tree not much lighter than water and barely able to float. For commercial production, the wood is kiln-dried for about two weeks, leaving the cells hollow and empty. The large volume-to-surface ratio of the resulting thin-walled, empty cells gives the dried wood a large strength-to-weight ratio because the cells are mostly air. Unlike naturally rotted wood, which soon disintegrates in the rainforests where balsa trees grow, the cell walls of kiln-seasoned balsa wood retain their strong structure of cellulose and lignin. Because it is low in density but high in strength, balsa is a very popular material for light, stiff structures in model bridge tests, model buildings, and construction of model aircraft; all grades are usable for airworthy control line and radio-controlled aircraft varieties of the aeromodeling sports, with the lightest "contest grades" especially valuable for free-flight model aircraft. However, it also is valued as a component of full-sized light wooden aeroplanes, most notably the World War II de Havilland Mosquito. Balsa is used to make wooden crankbaits for fishing, especially Rapala lures. Sticks of dried balsa are useful as makeshift pens for calligraphy when commercial metal nibs of the desired width are not available. Balsa wood is often used as a core material in composites; for example, the blades of many wind turbines are partly of balsa. In table tennis bats, a balsa layer is typically sandwiched between two pieces of thin plywood made from other species of wood. Balsa wood is also used in laminates together with glass-reinforced plastic (fiberglass) for making high-quality balsa surfboards and for the decks and topsides of many types of boats, especially pleasure craft less than 30 m in length. On a boat, the balsa core is usually end-grain balsa, which is much more resistant to compression than if the soft balsa wood were laid lengthwise. Balsa is also used in the manufacture of "breakaway" wooden props such as tables and chairs that are designed to be broken as part of theatre, movie, and television productions. The fifth and sixth generations of the Chevrolet Corvette had floor pans composed of balsa sandwiched between sheets of carbon fiber reinforced plastic.