Just farming a lot of expirience using furnace. Steps to reproduce:

1. Place furnace on top of the hopper.
2. Smelt something (i.e. iron ore)
3. Now, just spam-click at output slot in furnace (pick item before hopper get it), + it needed to move the maximum ammount of that item from your inventory to your clicked item (like does double-click on item usually).
4. Done, here is your expirience from [count of items on mouse] items.