The Kapok tree grows to 70 m (230 ft) with a trunk up to 3 m (9.8 ft) in diameter with buttresses. Kapok fiber is light, very buoyant, resilient, resistant to water, but it is very flammable. The process of harvesting and separating the fiber is labor-intensive and manual. It is difficult to spin, but is used as an alternative to down as filling in mattresses, pillows, upholstery, zafus, and stuffed toys such as teddy bears, and for insulation. It was previously much used in life jackets and similar devices until synthetic materials largely replaced the fiber. The seeds produce an oil that is used locally in soap and can be used as fertilizer. Native tribes along the Amazon River harvest kapok fiber to wrap around their blowgun darts. The fibers create a seal that allows the pressure to force the dart through the tube.