What is a key characteristic of microwaves?

Microwaves have a key characteristic of resonating with water molecules, which is fundamental to their use in various applications, particularly in cooking and heating food. This resonance occurs because microwaves can be absorbed by polar molecules like water, causing those molecules to vibrate and generate heat. This process is what allows microwave ovens to heat food quickly and efficiently by targeting the water content rather than by directly transferring heat from an external source.

In contrast, other options do not accurately reflect the nature of microwaves. Microwaves are not visible to the human eye, they do not have the highest frequency in the electromagnetic spectrum (as this is actually true for gamma rays), and they are not primarily used for heating metal, although they can interact with metal, it is not their intended primary application. Thus, the unique ability of microwaves to resonate with water molecules is what sets them apart and defines their characteristic use in both science and technology.