system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it matter which side is up or down when microwaving bagged popcorn?
gpt
The popcorn bag has some metallic looking material built into one side of it that absorbs microwaves better. It basically acts as a microwave activated hot plate.

Turning the bag upside down puts this heat source on top which is less effective for its use as a hot plate, and would shield the microwaves from reaching the popcorn directly.

You popcorn will still pop but not as evenly or quickly.