Click a cell and it becomes interior, and the six cells orthogonally around it become hull unless they are interior already. Click a hull cell and it becomes interior the same way, bringing its own ring of hull with it; click an interior again and it goes, taking the ring with it unless another room still wants it. The picture is the watertight skin of the hull: every part of a block's face that no other block is pressed against - so a room's walls are skin exactly as the outside of the ship is. Cubes only, no shapes yet.