STS stands for Silver Thiosulfate, a compound produced by a chemical reaction between Silver Nitrate and Sodium Thiosulfate. When applied to plants such as Cannabis, Silver Thiosulfate (STS) acts as an ethylene blocker. Ethylene can be considered the “female” hormone in Cannabis plants. By preventing the action of ethylene you can force a female Cannabis plant to develop and produce male-type flowers, pollen sacks, and pollen. Since this pollen was made by a female, the pollen can be used to pollinate another female plant creating feminized seeds which will be 100% female.