Click the green flag to reset. Then click on any block that is adjacent to the empty space to move it into the empty space. Repeat ad nauseam until the picture is thoroughly muddled up. Then try to unmuddle it without clicking the green flag! Alternatively press “j” to be set a puzzle to solve.
Inspired by those little toy puzzles with sliding plastic blocks. Here each block is a sprite, so 16 sprites altogether. Now made with cloned sprites - much less code! The picture was taken in Rendlesham Forest in the winter.