Sneakers (also called trainers, athletic shoes, tennis shoes, gym shoes, kicks, sport shoes, flats, running shoes, skate shoes, or runners) are shoes primarily designed for sports or other forms of physical exercise but that are now also widely used for everyday casual wear. Contemporary sneakers are largely made from synthetic materials, and the materials and manufacturing process produce, on average, about 14 kg (31 lb) of CO2 emissions. Some companies are trying to substitute more sustainable materials in their manufacture. About 90% of shoes end up in landfills at end of life.