I have learned, though, that the bacteria in the starter culture can be fussy when it comes to temperature. Boiling the milk kills other microorganisms that can cause spoilage, but you'll also kill the starter culture if you add it while the milk's still too hot, and if the milk's much cooler than 90 degrees at that point it will sour before it sets. It also took a little experimentation to find the right warm place in which to let those hungry bacteria get down to the business of eating sugars in the milk and producing the lactic acid that will, ultimately, take the pH level down to the point where the milk will thicken into yogurt. Setting my jar of inoculated milk on a sunny windowsill resulted in a fail, but when I set it in the same spot wrapped in a thick towel, the towel provided just the right amount of insulation, though it took more like 12 hours to make yogurt. When I had a convection oven that held at 100 degrees, that worked great, but most ovens won't go below 180. I've gotten good results heating the oven to 180 and turning it off as soon as I put the milk inside; the oven holds enough heat to get the job done (again, it takes a few hours longer). I've also had luck setting the jar outside in direct sunlight on a hot day, or in a nest of blankets on top of a heating pad set on medium heat. Whatever I rig up, I always strain the yogurt through cheesecloth once it's set, which produces a texture so dense and luscious I can't imagine going back to the store-bought stuff.