system
Given the following post, respond as if you're commenting on Reddit:
human
When we microwave food, why isn't the air in the microwave heated?
gpt
It's not just water molecules, it's polar molecules that rotate in response to the alternating magnetic fields.  This constant rotation is more akin to a vibration, and when molecules hit each other, they create heat.  

The majority of molecules in the air aren't polar (80% n2 19% o2, trace amounts of others).  Since they aren't polar, they don't rotate to align with the magnetic field and thereby generate heat.  However, the air does heat as a result of convection/conduction from the material that is polar.