My grandma told me to cook my roast in a 500°F oven for 20 minutes, and then turn the oven off and the roast will continue to cook.

 Your grandma doesn't like you very much, does she? Did she also mutter something that sounded a little like, "that's the last time you track mud across my clean floor?"

OK, yes. Your grandmother is right, the roast will continue to cook. You can also take the roast out of the oven after 20 minutes and it will continue to cook on the countertop — however briefly. But the point of cooking a roast, at least as far as we understand it, is to get the roast done, whatever done means to you.

The number of variables involved in your non-question are beyond our modest abilities to process. But here are the ones we can think of:

• How big is your roast?
• What shape is your roast?
• Is the roast at room temperature, refrigerator temperature, or in between when it goes into the oven?
• How heavy is your roasting pan and how well does it hold heat?
• Is your roast on a rack in the roasting pan?
• How quickly does your oven lose heat?
• How long does it take for the heat to reach the center of your roast?
• When do you consider the roast to be done — at 145°F in the center (medium rare), 160°F (medium), 170°F (well done)?

All of these factors play a role, and you can easily see that changing any of those variables significantly can wreak havoc with your roast. An 8-pound roast takes longer to cook than a 4-pound roast; your grandmother's method might leave one mostly raw while it passibly cooked the other.

We are indeed fans of the high-heat-to-start/low-heat-to-finish method of roasting, but the only real way to tell if your roast is done is by checking it with a thermometer. And the only way to keep it out of the temperature zone that government regulators consider to be unsafe (40°F to 140°F) for long periods is to keep the oven temperature at least at 200°F while the roast is cooking.

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