Final answer:
Water should not be used to extinguish electrical and grease fires due to the risk of spreading the fire or causing further damage.
Step-by-step explanation:
Water must not be used in trying to extinguish electrical and grease fires. While water is effective in extinguishing many types of fires, it can actually spread and increase the intensity of these two types of fires.
Electrical fires occur when an electrical circuit overheats or malfunctions, and water can conduct electricity, causing electrocution and further damage.
Grease fires, on the other hand, involve flammable oils and fats. When water is thrown onto a grease fire, it can cause the grease to splatter and spread, leading to a more dangerous situation.