Microwaves are absorbed by polar molecules, which are molecules with a positive and a negative end. The microwaves cause these molecules to vibrate, which generates heat. Non-polar molecules, which do not have a positive or a negative end, do not absorb microwaves and therefore do not heat up.

Some examples of substances that do not absorb microwaves include:

* Plastics

* Glass

* Paper

* Wood

* Rubber

* Metal

* Aluminum foil