People who are familiar with MREs know what it means!

This is the official visual instruction showing troops how to use an MRE heater. The MRE heater is a thin, square sheet somewhat about the size of an average man’s hand; it is a sort of “sandwich” of cardboard and some sort of water-reactive heat agent, which I assume to be magnesium or related to magnesium (it’s never occurred to me to look into it, really). It comes in a thin plastic bag: you put your food packet in the bag next to this “magnesium sandwich”, then pour in a bit of water, and lean against a “rock or something” so the water stays inside and the magnesium does its trick.

It takes maybe a minute or two to get hot (unless you have an old heater, in which case it may just get warm-ish or not even work at all) and it is a single-use item. A good heater can make the food pouch hot enough that you can’t easily touch it at first.

The concept of “rock or something” has become a symbol of unintentional comedy from the US military, which is usually not known for its sense of humor.

It seems to be the one and only place where the powers-that-be just shrugged their shoulders and admitted that troops are going to find whatever damn thing they need to get the job done, regardless of what instructions are put out. I can imagine the pitch meeting, where perhaps someone was trying to come up with some fancy heater stand device, when some impatient person on the committee looked his or her watch, realized it was getting close to dinnertime, and got up and said, “Just tell them to use a rock or something” and shuffled out to the parking lot.

Thus was the legend born.