Piston

Pistons are blocks capable of pushing most blocks, depending on the direction they are facing.

Functionality

 * Pistons when placed always face the player.
 * Pistons are redstone triggered. When powered, their wooden surface extends outwards by 1 block, pushing up to a maximum of 12 blocks.
 * When going back to the unpowered state, it does not pull back any block. See Sticky Piston.
 * Its wooden surface can also push players, mobs and other entities on the floor by 1 block if they are in the way. However, if there is no space for the player/mob/entity to be pushed, the extended piston head will just pass through them without harm. The piston can still push a mob or player off a one block wide walkway, which can be coupled with a Pressure Plate for a fairly effective trap.
 * The extended piston head is considered solid and transparent (just like Glass). If a piston head extends into a player/mob this way, they will not suffocate, and the player/mob can move out of it but not back into it.
 * Similarly, if a block pushed by a piston is pushed into a player/mob, the player/mob will get pushed if there is space. If there is no space, the block will start to occupy the same space as the player/mob and cause suffocation if the block is opaque.
 * When blocks are pushed by a piston, they are momentarily non-solid and cause any players/items on the moving block to fall through the moving block.
 * Pistons can be used to toggle fluid flows like a flood gate by extending into or retracting from the space where the fluid would flow through.
 * Pistons can push Minecarts and Boats. Note that if the minecart is on a rail, the rail will also get pushed.

Pushing blocks

 * Some blocks cannot be pushed by Pistons and the piston head will not extend:
 * Obsidian, Bedrock to avoid abuse
 * Blocks with extra data (tile entities) attached - note block, chest, furnace, dispenser, monster spawner.
 * Pistons will not push blocks into The Void or the top of the map.
 * Extended pistons (both sticky and normal) because moving blocks are temporarily stored as tile entities.
 * Portals
 * Pumpkins, Cobwebs and Jack-O-Lanterns are turned back into items when pushed. (If trying to create extendable/retractable lighting, you can use Glowstone instead.)
 * Melons will turn into slices if pushed by a piston.
 * Pistons cannot push water or lava, only obstruct it. A piston head extending into a water or lava source block will destroy the source block.
 * Pistons cannot push most blocks that are "attached to a block” such as levers, torches, redstone, signs, etc. They will be detached and drop as an item. Exceptions include:
 * Trapdoors, as long as their hinged side remains attached to a solid block in their new position.
 * Rails (and Powered Rail, Detector Rail) as long as they remain on top of a solid block in their new position. Rails will re-orient themselves after being pushed just as they do when placed manually.
 * Blocks that are attached to pushed blocks will be broken off unless the block which is now under/behind them is also acceptable; for example, torches can be placed on a "self-repairing" cobblestone structure.
 * Pistons cannot push a chain of blocks more than 12 blocks long, or else they will not extend.

Powering Pistons

 * Pistons can be powered horizontally through single blocks by way of either redstone or repeaters
 * Pistons can be powered by a torch directly adjacent to them (above, below, or to the side)
 * Pistons can be powered by a torch from below when said torch would power the block the pistons are resting on (just like powered rails)
 * Pistons can be powered by any powered block directly adjacent to them, and by any powered block one block above and to the side. However, the piston won't recompute its state unless a block directly next to it is changed. An adjacent piston retracting causes a recompute, but an adjacent piston extending doesn't.
 * Both redstone dust and repeaters will power the piston directly in front of it as well as a second piston below this piston. This allows a wall made completely of pistons to be constructed. Redstone dust will also power the block beneath itself which in turn will power the third piston below the first two, but due to the bug mentioned above it won't recompute it's state.
 * When toggled between on and off rapidly with a gravity affected block (such as sand) above it, a piston will eventually break the block, which can be picked up as a resource. However, gravel will never break into flint.
 * A repeater cannot transfer power through a piston.

Common Usage

 * Pistons are commonly used to push/pull blocks to simulate opening and closing doors, gates and to create traps.
 * Pistons can be used to make self-repairing bridges by using cobblestone from a cobblestone generator. The same principle can be used to make a self-repairing house.
 * When used in conjunction with redstone repeaters, pistons can make a redstone piston clock capable of almost any speed.
 * Pistons can be used to make traps, as pushing a block into a player will suffocate him. Their advantage is that they are easily resetable, and are very subtle.
 * A piston powered clock linked up to a square of blocks designed to be pushed round in a loop can create an endless loop of data. Combined with the inability of transparent blocks (such as glass) to transmit power, a redstone repeater can be run into the pushed data block to optionally perform a function on the other side. Scale this up into 3 dimensions and you get an array of data looping round, being tested to see if it transmits the power or not. This can be used to create redstone sign outputs, to play musical sequences, and a countless list of other things. A single cell can also hold at least five different states as opposed to standard two.
 * Another use for Pistons is to put a block on top an upward facing piston, with a track on top of that, then have a minecart one block away from it on the track so that when you activate the Piston it will slant the track and get the minecart started.
 * Pistons can be used to move blocks like glass or ice that cannot be picked up. However, pistons cannot push obsidian.
 * Pistons can be used for semi-automated farming. Using a sticky piston with a dirt block or similar solid block attached, the piston is capable of pushing wheat or sugarcane off soil- the blocks will subsequently turn it into their respective items. Then replant manually.
 * Pistons can be used to build downward from a platform in the air by placing it on a block above you, and pushing blocks downward into place.
 * Pistons can be used to build opening and closing windows.

History
The original piston was a mod posted on the Minecraft Forums by Hippoplatimus. The code for that version was given to Jeb, who then worked on shortly implementing them into vanilla Minecraft. However, due to his work on the Pocket Edition, pistons were delayed, and were not released for the 1.5 update nor for the 1.6 update.

Finally, when the Pocket Edition was good enough to show at E3, Jeb went back to work on the PC version of Minecraft and pistons were finally added in version 1.7, including Sticky Pistons.

Bugs

 * If pistons push a block rapidly, and you break the block, you will get Block 36, or the Piston Arm if you break a block near the piston's arm.
 * An extended piston will not retract if one of the blocks diagonal to it is powered.
 * If two extended pistons are one above the other, the lower one cannot retract until the upper one retracts. If the top one will start extending while bottom one is still retracting, the last will start extending as well.
 * If a sticky piston would pull a block back and another piston is waiting to push out to the spot that block would occupy after being pulled, the block is deleted.
 * Powering pistons with a redstone torch in some cases seems to always consider the torch as "on", even if powered off.
 * Rear part of a piston during retracting has no collision: when pistons point down it is possible to glitch through them when they retract, it is also possible to glitch through a retracting piston while walking against it.
 * Blocks being pushed by a piston have no collision in certain places.
 * When a sticky piston pulls a door, it becomes a half door attached to the piston.
 * A piston attached to a sticky piston may disappear when extended into flowing water.
 * Light passes through pistons, both normal and sticky, powered and unpowered of both types.
 * If a sticky piston is pulled down by another sticky piston, it will not pull the block it is attached to with it.
 * When an upward-facing piston attempts to repeatedly push a stack of gravity-affected blocks upward, it will destroy the blocks without dropping them to be collected.
 * Double piston can be created using unknown methods, The side piston can only be powered from above and if its powered from beneath it will retract and vanish, this bug seems to break SMP servers, therefore no tutorials are available yet.
 * Pistons can get stuck on and off simultaneously. The only way to fix this is to destroy the piston and replace it.
 * When a piston or a sticky piston is placed upwards (with the redstone input being located 1 block bellow) and a rapid pulse signal is applied to the redstone, if the player tries to break the piston or sticky piston with any item, block, tool, or hand, the piston or sticky piston will break, dropping an item, but leaving a breakable piston head in mid-air. Click the following reference to see this bug.
 * If a Piston is facing upwards and a gravity-effected block (such as Sand) is put on it (if using a Sticky Piston, any block works) and Redstone Dust is put near it, and the player puts a Redstone Torch on top of the Redstone Dust, then the player gets an extra Redstone Torch. However, this only works if the player if the Redstone Dust is on the North or South sides. This is demonstrated in this video.
 * Although Sticky Pistons can also move Minecart Rail up like normal ones, retracting the Piston turns the rail back into an item. It is advised to use normal Pistons for rail movement to avoid losing them when the Piston is turned off.

Trivia

 * If a piston pushes a gravity-affected block over a hole, the block will fall, even if attached to a sticky piston.
 * Pistons cannot be used to 'crush' players or mobs against walls. A piston will simply pass through them if this is attempted. However, blocks being pushed by piston will crush players and mobs and suffocate them, making it useful for traps.
 * If attached from up above, pistons may be able to push blocks downwards and suffocate the player or mobs.
 * If a water spring is placed on top of a piston, it will disappear if the piston is extended. This also applies to lava springs.
 * Pistons can be placed near cacti so that when they are activated, and if the pushing end is adjacent to the base of a cactus, the cactus will break.
 * Pistons will break leaves when pushed.
 * If you push a burning block, the block will be put out. If you push a row of burning blocks, they will all stay on fire except for the farthest one.
 * Pistons cannot push any block into level 128.
 * Pistons can be used to create logic gates. These may be more space efficient than standard gates and clocks created from them can also operate at much higher speeds without burnout.
 * Pistons may also be used the same way one-way or minecart-only soul sand gates work by putting an extended piston one block above a minecart track allowing only small or tiny slimes, cave spiders, and chickens to enter. Doing this keeps pursuing mobs out (except for slimes). When a minecart passes through the player will not be suffocated and continue going. Alternatively they can be used to make entire 2 block high doors for minecarts on a slope, since if a block is put adjacent to the theoretical hypotenuse of the slope it will touch the rails, but not cut them off and will allow no space for any mobs at all.
 * Pistons can be mined with bare hands in less than 2 seconds.
 * Pistons Share the #1 spot with cake for crafting complextiy, each requiring 4 individual items to make the item itself. However, Pistons have a much shorter crafting chain, requiring at minimum having crafted an iron pick.
 * Pistons do not work at the lowest level of bedrock (or the level above the Void).