You are supposed to put some water in the heater packet and then lean the MRE pack against, famously, a rock or something, while it warms up. The reaction shouldn't take more than a minute.
What, young soldiers with little to do have doubtless asked many, many times, would happen if you were instead to collect the powder from several ration heaters and put it in a sturdy sealed vessel, with some water?
Why, this would, thanks for asking!
Unlike relatives like the dry ice bomb, the heat of these "MRE bombs" will soften plastic, allowing the impressive inflation seen above.
If this just isn't dangerous enough for you, you could always use a metal canteen instead of a plastic bottle!
Note also the suggestion of putting the MREs' Tabasco sauce in the bomb, to add a chemical-warfare tang to the operation.