Most self-tanning products contain the chemical dihydroxyacetone (DHA). DHA is a colorless sugar that causes a chemical reaction when it comes into contact with the dead cells in the top skin layer. Specifically, skin is made up of two main layers, the epidermis and the dermis. The epidermis is on the top, and at the very top of this top layer is a thin layer called the stratum corneum, also called the "horny layer." It is the dead cells in the stratum corneum that are stained by DHA -- You lose millions of cells from the stratum corneum every day. The staining from the reaction lasts about 5 to 7 days because the dead cells that are stained fall away and are replaced by new cells continually. Hope that helps answer your question!