Hoppers can't take water bucket out of furnace

1. Place a furnace and a dropper/hopper facing to the furnace next to it
2. Put an empty bucket in the hopper/dropper
--> It won't get transported in the furnace-

1. Place a hopper and above it a furnace

2. Power the hopper

3. Place a lava bucket in the fuel slot of the furnace and a wet sponge in the upper slot (don't know the name right now)

4. When the sponge is dry, remove the redstone signal powering the hopper
   -> The hopper doesn't take the water bucket