Curly hair is caused by genetics, of course, but it’s also caused by some basic chemistry. Ringlets, kinks, and waves arise when pairs of sulfur atoms in the amino acid cysteine link up together, forming, appropriately enough, disulfide bonds. (There’s a lot of cysteine in hair because it’s one of the major building blocks of keratin, the protein that makes up most of the hair shaft.) During this process, each of the sulfur atoms sheds an electron, and that’s what allows one of the most reliable techniques for straightening hair to work.

During Japanese hair straightening, a reducing agent—in this case a chemical known as thioglycolate—forces electrons back onto the sulfur atoms, reversing the process that created the bond. After the hair is nourished with shampoo and a protein-rich conditioner, the stylist will apply a solution of thioglycolate that’s activated by the heat of a dryer. A flat-iron then makes hair as straight as possible before a chemical neutralizer tells the thioglycolate to stop processing and keeps the hair in its new shape.

The process often takes three to four hours, and clients shouldn’t style or wash their hair for about three days afterward to avoid disrupting the treatment or losing that nice salon smell sooner than necessary. The chemical reaction is permanent, and changes the actual structure of the hair—that’s why Japanese-straightened hair won’t curl again the way keratin-smoothed or flat-ironed hair eventually does, at least until it grows out. At that point, if you want to maintain the sleek look, you can go back to the salon to touch up the roots.