system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How microwaves work, such that my leftover gravy boils on my plate while my lump of cold mashed potatoes stays cold?

Eating leftovers, obviously.
gpt
Microwaves are at the right frequency to cause water molecules to vibrate, increasing the temperature of the food. Gravy contains more water than mashed potatoes, which are relatively dry, so gravy heats up much faster.