You can make popcorn in several different ways, including on the stove, over a campfire, or perhaps the most common way, in the microwave. If you make too much popcorn, or are looking to heat back up some popcorn that you brought home from the movie theater, you can achieve nearly the same experience by reheating the already popped popcorn in the oven.
Preheat the oven to 250 degrees Fahrenheit, and line a baking sheet with aluminum foil while you are waiting.
Coat the foil lightly with cooking spray or a butter spray. This will help give the popcorn more flavor as it is reheating.
Cover the foil with the popped popcorn, and place the baking sheet into the oven. Check the popcorn periodically, making sure the popcorn doesn't burn. The popcorn will be ready when it is warm. Warming the popcorn should take no longer than 10 minutes.