Grease is an oil, it's a liquid at burning temperatures.
Oil and water don't mix, so when you squirt water on grease you get balls of water in oil and vice versa. The balls of water in oil are a big problem. The oil is over 212˚F (100˚C) so the water boils into steam. The steam expands, spreading the oil into a big, thin sheet, before it pops. Now you have a big, thin sheet of burning oil and the pop sends it flying through the air. Super-bad.
Oil is less dense than water, so oil floats on top.
When you add water to a hot pan covered in oil, the water slips underneath the oil, is quickly heated to boiling temp. When water boils, it becomes steam.
Steam takes up more space than the liquid water, so it forms a bubble under the oil. This bubble fills up with more and more vapor until it pops, spewing oil all over the place.
It doesn't matter whether the oil is on fire or not. The same thing will happen, except then the oil being spewed all over the place by bursting bubbles is *on fire*.
When the flaming oil is airborne it is in contact with oxygen on all sides. Fire consumes oxygen, so this big increase in available oxygen causes the fire to flare up even bigger. Trying to put out a grease fire with water is one of the top reasons of buildings burning down worldwide.
Instead of adding water, put a lid on the pot. The fire will consume all the oxygen inside the covered pot and then burn out.
Grease and water do not mix. Try it, pour a bit of cooking oil into a cup of water and you'll see it congeal into globules. No mixing will occur. When grease is on fire the water does not mix with it either. The source of the flame will not be touched by the water. You might splash out a flame or two but the grease will remain untouched as will the flame.
Using water to douse a fire is an attempt to prevent access to oxygen enough that the reaction doesn't sustain itself. When you have an oil fire, you have things working against you.
Most oils are less dense than water, so almost all of the water you pour on it will end up underneath the oil leaving the oil in contact with oxygen and it will continue to burn, but now you have more liquid, so it is more likely to spread around.
Fire causes materials close by to get hot and oil can heat well above the boiling point of water, so if the oil is hot enough, the water will boil and the escaping steam will pop from the surface of the oil, spreading droplets all over the place which burn very well with all of that surface area and being inside of the flame.
Since it isn't likely to be effective and increases the risk of the fire spreading, it's usually better to find a different way to extinguish the flames, move the fire to a safe location, or evacuate the area.
When you pour water on an oil fire the water mixes with the hot burning oil whilst simultaneously being vaporised. These two processes happening simultaneously results in an explosion with burning oil being fired in every direction. This is why you should smother an oil fire with a wet cloth.
The reason why the flames increase, is because the grease will float to the top of the water, and because of the water, the grease is now flowing, catching other things on fire.
The reason water usually puts out fires is it lowers the temperature of the fuel by absorbing heat very rapidly or smothering the flame by depriving it of oxygen.
With a grease fire, water is useless. The water does not mix with grease, so it just sinks to the bottom of the grease puddle where it doesn't absorb very much heat due to the lower surface area. It can't smother the flame either because it sinks to the bottom of the pan. This is why it doesn't put the flame out.
The problem is made worse by the fact that grease has a higher boiling point than water. This means when you pour water into flaming hot grease, the water violently boils and throws flaming balls of grease everywhere. With just a grease fire, you have a limited surface area for the grease to catch fire. When you add water, it increases the surface area of the grease, allowing for a bigger ball of fire. [Here](https://youtu.be/Fj5ex0cDTUs?t=113
) is a good example of what I'm talking about.
For two reasons. Differences in density, and hydrophilic versus hydrophobic.
Oil is lighter than water and floats on top of it. It is also hydrophobic meaning it does not mix with water. So pouring water on an oil or grease fire causes the burning oil to float on top of the water and continue to burn.
Imagine a small grease fire. The amount of burning grease exposed to the air is relatively small. Only the exposed surface area of the grease burns. Now add water. The water increases the bulk which increases the surface area of the oil, which allows more of it to burn than before, making a larger fire. The hot oil will also often pop in the presence of water as the water flashes to steam, exploding bits of grease and oil upwards, mixing even further with the air and causing a flash over burn.
Grease or oil has the capability of being heated, and staying liquid at temperatures way hotter than water boils, like over double the temperature. And that's before it begins burning. When it's burning the liquid oil underneath is going to be stupidly hot.
A given amount of water increases in volume by **1700 times** when it turns to steam.
Water is denser than oil, so it pours in, and goes straight under the surface, and is almost instantly flash-boiled to steam by the 200^o C oil. The steam, expanding suddenly (pretty much exploding), blows the oil everywhere. Oil that's in small droplets burns *far* more effectively, and you've just turned most of the pot into small droplets, in the vicinity of an already burning fire.
The result is a massive fireball.