system
Given the following post, respond as if you're commenting on Reddit:
human
How come metal sparks in the microwave yet the insides of microwaves are made out of metal?
gpt
Roughly: the microwaves induce an electrical potential in the object metal, this causes sparks as differences in the potential across the object metal causes arcs of electricity (especially with water vapour that may come from the cooking food). The side walls of the microwave are earthed, so any potential induced by the microwaves won't build any charge.