It is up to you to stack the blocks in such a way that they remain firmly stacked long enough to pass the level.
This is easier said than done, because there are some pretty awkward shapes for you to fit in to pile. Just make clever use of other blocks to stabilize the unsteady ones and you should be able to keep everything in place.
Stack the blocks into a steady pile.
None of the blocks may fall off the screen. Once you place the last block a timer starts that needs to expire before you can go to the next level.
Left click to let the block go.
