system
Given the following post, respond as if you're commenting on Reddit:
human
How can water extinguish a fire, but can make a grease fire worse?
gpt
When extinguishing a regular fire, water does two things: It cools down the burning substance and it smothers it, depriving it of oxygen. Both of these things work well because the water forms a thin layer on the surface of the burning object / substance.

But when you throw water on burning grease or oil, something else happens. Since oil/grease is hydrophobic and less dense than water, the water will pass through the oil and sink to the bottom. You then have the water trapped by a layer of burning oil that is hotter than the boiling point of water. The water is rapidly brought to boil and when that happens it expands to well above 1000 times its volume as it turns into a gas.

This rapid expansion quickly pushes the oil away, breaking it up into tiny droplets. In the form of droplets, the oil has far more surface area than what it had before, giving it much better access to the oxygen in the air and therefore speeding up the rate of combustion.

So instead of a contained volume of burning liquid with limited surface area exposed to the air, you suddenly have a cloud of burning droplets, which are burning much more intensely than before.