Minecraft wiki entry for Slime_Block, associated text: When a slime block is pushed or pulled by a piston, it attempts to move all adjacent blocks in the same direction. The types of blocks that can be moved are the same as those that can be pulled by a sticky piston. Blocks that cannot be pulled by a sticky piston (i.e. all the blocks listed on the table on the pistons page) stay in place. The blocks that are moved may in turn push other blocks. For example, a slime block sitting on the ground attempts to move the ground block underneath itself, which pushes additional ground blocks in the direction of motion just as if it were being pushed directly by a piston.