Fibers can be natural or synthetic. Natural fibers include cotton, bast fibers like flax and hemp, wool, and silk. Synthetic fibers are man-made and include nylon, polyester, acrylic, and rayon. Cotton fibers are soft, porous, and absorbent. They have low thermal conductivity. Wool fibers have a crimped structure and can be dyed with acid or reactive dyes. Silk fibers are very fine and smooth with triangular cross-sections, but have low UV light resistance. Common synthetic fibers each have their own properties suitable for various applications like clothing, parachutes, and tires.