Hydroxy acids are derived from natural things like fruit peels, sugar cane, and bark. These acids work to “loosen the glue that holds dead skin cells together,” as one MD put it. By doing this, they clear away old stuff from your skin to leave you looking brighter and feeling tighter.  Products come with different concentrations of these acids. Everyday serums or creams will likely have a lower concentration for gentler exfoliation, while once-in-a-while peels amp things up to cleanse deeper. The FDA suggests using an overall concentration of less than 10% daily.