An explanation of how something works is a
description of a process, showing A then B then C. 
It is important to include only essential information in an explanation. Non-essential information draws the reader's attention away from the explanation, causing confusion.
Here are two attempts to explain how a fire extinguisher works:
Example 1:
A fire extinguisher is an important safety device. Fire extinguishers are often red, although they may be chrome, cylinders. Attached to the cylinder is a pressure gauge, a handle, and a spray nozzle or hose. When the handle is pressed, the fire extinguisher expels water or fire retardant material, putting out the fire. Fire extinguishers are found in many homes, and are often required in businesses and commercial industries. Every home should have one!
Example 2:
A fire extinguisher works much like a spray can. The fire extinguisher is a cylinder that contains water or some fire retardant material. Also inside the extinguisher is a second, smaller cylinder about the size and shape of a test tube. This small cylinder holds carbon dioxide gas that is under very high pressure. Finally, the extinguisher contains a long tube that runs from the nozzle at the top to the bottom of the larger cylinder, something like a straw in a glass.
When the handle on the fire extinguisher is pressed, a valve on the top of the interior cylinder opens, allowing the carbon dioxide gas to fill the upper portion of the extinguisher. This causes the extinguisher to be pressurized like a can of soda that has been shaken. The water or fire retardant material is forced down by the pressurized gas and goes up through the straw-like tube and out the nozzle.
The material squirting out of the fire extinguisher smothers the source of the fire, depriving it of oxygen, causing the fire to go out.
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