Minecraft wiki entry for Piston, associated text: Pistons extend if powered, but it's notable that they can take power from the block above them – that is, even if the block is air, if that block would have been powered, then the piston is powered. They can also be powered through their extended piston, which produces several useful quirks that are commonly used in circuits. A piston can push up to 12 blocks when it extends; however, a sticky piston can pull back only one block when it retracts. Some blocks are immune to being pushed or pulled, notably obsidian and any block with a GUI or inventory (except crafting tables). Other blocks drop as items if a piston tries to push them; this includes most "attached" blocks such as either sort of torch, or doors, but also a few others such as jack-o-lanterns.