Laws of Shabbat - 12

It is forbidden to place any food in a vessel that is on top of the fire, whether it is liquid or solid, whether it is cooked or not, and whether it is cold or hot. Therefore, you should not put anything, not even salt, into a pot sitting on the fire. Furthermore, you should also not put anything into a water heater, which also sits on fire, for it is considered a vessel with which to heat up food.