There was an invention that allowed you to boil water by putting a bag of chemicals in a bowl with some water and/or food. I don't remember its name and I would like to use it while hiking. (other hikers may know)
Such packets are included in MREs (Meals, Ready-to-Eat) and are referred to as "flameless ration heaters". The packets contain finely powdered magnesium metal, alloyed with a small amount of iron, and table salt. Adding a small amount of water results in a salt-water electrolyte fluid that allows the magnesium and iron to rapidly react and generate heat.
The standard use is to place the food packet inside a bag along with the heater packet, then add a small amount of water. The top of the bag is folded over, the bag is held level to allow the heater to soak up water, and finally the bag is placed back in the carton and left at a slight incline while the food warms.
Note: Per the instructions, you should not drink the water or use it in food items. Also, the reaction produces hydrogen gas, so be cautious of breathing the fumes or using it in a confined space or near open flames.
The packets weigh about 1.5oz each and are single-use, so it's usually more efficient to use traditional cooking methods (alcohol, isobutane canisters, etc.) for trips that will require more than one hot meal.