Fillers are chosen on the basis of comfort, resilience, thermal properties, cost and also for medical and ethical reasons. The most common synthetic fillers are materials derived from polymer fibers, such as polyester and memory foam. Synthetic fillers in general are inexpensive and in the case of memory foam can retain their form longer. Natural fillers have been used since antiquity. The most common are feathers, down, wool, latex, cotton(particularly in India), and buckwheat. More exotic materials have also been used including straw, wood, or stone. Down is usually the softer filler and offer good insulation, but is more expensive due to its relative scarcity. Down has been known to be plucked from live geese, but there are already cruelty-free certifications for down products. In India, traditional pillows are made with kapok, the fluffy, glossy fruit-fibres of the trees Ceiba pentandra and Bombax ceiba.