nothing, but nothing thickens fine hair. some products lift the cuticle and leaves it sticking out like a pine cone instead of flat to the hair like a snake's scale, which gives the impression that the hair is thicker.....
Exactly woofgang. There are lots of human products that do that like medicuticles and niaoxin . Hairdressers say that this can lead to sun damage as the cuticles are open. If you have short hair and cut it often it is not a problem but with long hair it isn't such a good idea.