Minecraft wiki entry for Piston, associated text: The moving piston is an unobtainable technical block that contains part of a piston head, and/or part of one or two blocks that the piston is carrying into or out of the grid cell (including blocks carried indirectly via slime blocks). Since moving blocks vary in how much of each grid cell they occupy, they can't be stored as normal blocks and are instead stored as block entities. It is overwritten with air, the piston head or the carried block at the end of the piston stroke; but if it is placed through editing and no piston is connected, it remains indefinitely.