Fire Extinguishers

In an office building today, I observed four bright red buckets of sand hanging on a rack with the words Fire Extinguisher on them.  I thought, wow that's primitive.  But then I gave it more thought and questioned whether perhaps it's brilliant instead.  For example... how do I know if there is anything inside a traditional fire extinguisher.  I've tried shaking one before and still couldn't tell. But I can see the sand is right there!  How do I know if the propulsion of the traditional extinguisher is going to work?  All I need is my arm for a bucket of sand.  You decide...