My first encounter with MRE heaters was in the Marine Corps. We occasionally used them to heat our MRE’s, per the official purpose, but mostly we used them for fun. MRE heaters, or FRH, or flameless ration heaters, are made from iron, magnesium, and salt. They react with water as a catalyst to heat it to a boiling point. A by-product is the offgasssing of hydrogen. That’s the science-ey part.
Here’s the fun part.
The heaters are intended to be used in an open pouch in open air where the offgassing and heat pose no threat. If, however, you place the heater and water catalyst in an enclosed, non-permeable, flexible closed space (i.e. 16-20 oz. plastic soda bottle), the heat and the hydrogen gas will cause the bottle to expand until it explodes. It’s a small explosion, non-schrapnel producing, and takes about five minutes to create. Basically, it just splatters water all over the place. I wouldn’t hold the bottle, but you can stand within 10′ of it and be okay.
We used to set these up and roll them down beside someone’s sleeping bag or under a cot to create a bit of an alarm. As I said, great fun.
It only takes one heater to blow out one bottle. To give my boys a little blast I placed five of them into the bottle. All you have to do is remove the powder, making sure to not ingest any of the dust. Drop it in the bottle. Add water. Tighten the lid and shake. Then place it and walk away. It’s even louder if you drop it into a dumpster or bucket, but you don’t get to watch the bottle expand from the heat if you do it that way.
You can buy MRE’s pretty much at any camping or army surplus store. Just be careful where you blow these guys. They sound like a gunshot and could get you into trouble. If you’re playing Michael Westen for some reason and need a timed diversion, it makes an expedient decoy.