The game has skull blox that you can't remove by clicking. It also has eater blox that will remove all blocks including skulls in the direction they face. There are rotators that will change the direction of the eaters. Finally there are magic mushrooms that will change the color of surrounding blox.
It all adds up to a fun and addicting game that builds excitement and adrenaline when the stacks start to get away from you and reach for the top. At that point fast thinking and reflexes take over in a flurry of frantic clicking.