Preheat a heavy casserole over medium heat for a few minutes. Add the popcorn kernels and start to heat, stirring frequently. Once the kernels start to pop, reduce heat to medium-low and cover. Let the corn continue to pop, shaking the casserole a few times as it does, until you hear the sound of popping practically stop. Remove the popcorn from the heat and uncover. Transfer to a large bowl or baking sheet and let cool.