Hair conditioner is a hair care cosmetic product used to improve the feel, texture, appearance and manageability of hair. Its main purpose is to reduce friction between strands of hair to allow smoother brushing or combing, which might otherwise damage the fibres. Because the hair shaft is made of dead cells that cannot renew themselves, conditioners do not repair hair biologically; they deposit material that coats the fibre and fills damaged areas of the cuticle, leaving a smoother surface. Various other benefits are often advertised, such as hair repair, strengthening, or a reduction in split ends.