Popcorn can be cooked three different ways: using a microwave, an air popper, or on the stovetop.

Microwave popcorn cooks exactly the same way a stovetop popper works. The bag contains a metal plate which is heated by the microwaves. That melts the shortening and fries the popcorn. That's why the bag is marked "THIS SIDE DOWN".

An air popper does the same process except without oil. Heat is transferred into the popcorn by hot air.