Minecraft wiki entry for Redstone_components, associated text: When activated, a piston pushes the block in front of its arm, and up to 11 more blocks in front of that (up to 12 blocks total). When deactivated, a regular piston pulls its arm back (leaving an air block in front of the piston), while a sticky piston pulls back both its arm and one block (leaving an air block on the other side of the pulled block).