Answer:
The man who invented it doesn't want it. The man who bought it doesn't need it. The man who needs it doesn't know it. What is it?
Though easy to spot, when allowed to plume, It is hard to see, when held in a room.
Someone walked into a room and you heard a crash! Someone died. What happened?
Double my number, I'm less than a score. Half of my number is less than four. Add one to my double when bakers are near. Days of the week are still greater, I fear.
I dig out tiny caves and store gold and silver in them. I also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs my help. Yet many people are afraid to let me help them.
I repeat only the last word you say. The more I repeat, the softer I got. I cannot be seen but can be heard. What am I?