When you have charcoal (smelt wood) and coal (found in coal ore) and put them into a chest, they do not go together. But, if you put them both in a furnace together and take them out again, they are combined and either the coal becomes charcoal or the charcoal becomes coal.

1. Have charcoal, coal, a chest and a furnace ready.
2. Put them (the charcoal and coal) into the chest. They should not combine.
3. Put them (the charcoal and coal) thgether into the furnace. They should combine.
4. Put them (the charcoal and coal) back into a chest and walk away happy that you conserved space.

• The Bug -
  When you put charcoal and coal into a furnace, they combine.