Epsom salts are the common name for magnesium sulphate — the bitter, crystalline salt named for the Surrey town whose mineral waters contained it. Dissolved in bath water, it is the traditional soak for aching muscles; taken internally in measured doses, it acts as an osmotic laxative, drawing water into the bowel. The magnesium is the point in both uses — though skin absorption from baths is modest, oral magnesium sulphate reliably corrects the laxative need. Food-grade magnesium sulphate also serves as a firming agent and mineral source in brewing and tofu-making, where it coagulates soy milk in some traditions. The name survives from the spa era, when “taking the waters” meant dosing with the local minerals. Like all laxative salts, it demands respect for dose — the line between remedy and purge is measured in spoonfuls.