Any serious baker can back up this fact: Don't open the oven door when cooking at home. When you get your oven preheated to the temperature you need, place whatever you're roasting or baking inside, and leave the door closed. When you open and close the oven to check on your food, it cools the inside by about 25 degrees each time. That not only wastes energy, it increases the overall cooking time and can change the outcome of whatever it is you're cooking. Just flip on the light switch and peek through the window to check on your food.