I maintained that temperature for 10 minutes ( according to McGee 's instructions ) before moving the pan to a cold water bath and lowering the temperature to 115 degrees , at which point I mixed in 3 tablespoons of our favorite commercial yogurt from Seven Stars Farm . I poured the mix into a warm quart jar and placed the jar in a cooler with a heavy pot full of hot water . I placed an instant - read thermometer in the cooler , aiming to maintain a temperature of about 115 degrees . Turned out I did n't need to do anything further .

Question: Why did they place a thermostat in the cooler ?
It was important that they had the temperature perfect .