You may recall that recently we had to rescue a sheep from the fence, her head was stuck.  Once my wife and I freed the sheep, I found this on the ground:

Dave Winsor

I thought I was RICH, you know, that this was some rock from outer space that just happened to land in my field.  My wife reminded me that we'd seen this before.  I chose to harbor my dreams of instant riches.  But my Facebook friends Al and Rick BOTH knew what it was:

Clinker Coal!  It's COAL.  Not space rock, not worth anything except hours of daydreaming of buying a tropical island with my find.  Nope.  Coal.

Definition courtesy of the Free Dictionary:

1. The incombustible residue, fused into an irregular lump, that remains after the combustion of coal.
2. A partially vitrified brick or a mass of bricks fused together.
3. An extremely hard burned brick.
4. Vitrified matter expelled by a volcano.

5. Slang

a. A sour note in a musical performance: hit a clinker.
b. A mistake; a blunder.
c. Something of inferior quality; a conspicuous failure: a clinker of a show.
I have covered most of these definitions!