Box Blocks

You find a box in the attic.

In this box, you find a pile of wooden blocks, painted in different shades of brown.  If you arrange the blocks into the shape of a box, then the blocks will transform into a box of that shape.  Opening that box will reveal it to be empty.  You can place any item into the box so long as it will fit.  Closing the lid will transform the box back into a pile of blocks.

Reforming the blocks into the same arrangement will bring back the previous box with the item which you placed within it.  Arranging the blocks into a different order will form a different box which may or may not be empty.