Balsa wood is the softest wood, easy to cut and shape with the simplest of hand tools.  My first memories of Balsa wood are of the light airplanes with rubber band driven propellers, that we used to buy for a quarter back when I was a kid.

Source

http://en.wikipedia.org/wiki/Ochroma_pyramidale

Balsa (Ochroma pyramidale, synonym O. lagopus) is a large, fast-growing tree that can grow up to 30m (98ft) tall. Balsa trees are native from southern Brazil and Bolivia north to southern Mexico however Ecuador has been the primary source of commercial Balsa. In recent years some Balsa has been plantation grown.

Balsa wood is used to make very light, stiff structures in model bridge tests and for the construction of light wooden aeroplanes, most famously the World War II de Havilland Mosquito. It is also used in the floorpan of the Chevrolet Corvette Z06 sandwiched between two sheets of carbon fibre.

Keith