I have discovered an ingenious way to turn one measly little stack of coal into a whopping 8 stacks (no HAX!)
What you will need
Coal
Tree farm (or forest)

Step 1:
Go get some coal. A stack will do.
Step 2:
Go chop down trees, 3-8 stacks. Make sure to replant
Step 3:
Go to your furnace. In the bottom (heat source) place the coal. In the top, place the LOGS.
Step 4:
Wait. When the furnace is done, take the charcoal out and put in more logs.

How does this work?!?
The coal will turn the logs into charcoal, the same thing as coal. It will burn 8 per unit (just like coal)! This was implimented a few updates ago, along with note blocks and cake, it was just hidden. Notch created this for those helpless crafters that seem to have a world with very little coal (easy first night torches)

Hope you learned : D