Gail Borden invented condensed milk after witnessing children die from contaminated milk on a transatlantic ship. He sought a way to make milk stable and safe. By boiling milk in a vacuum, he could evaporate the water at a lower temperature, preventing it from scorching. He then added a high concentration of sugar, which acts as a preservative by drawing water out of bacteria cells (osmosis), effectively killing them. This 'everyday' pantry staple became famous during the American Civil War when it was used as a high-calorie, shelf-stable ration for soldiers. It later became an essential ingredient in desserts like Key Lime Pie and Thai Iced Tea.