TNT

TNT acts like a normal block in Creative, but in Survival it behaves like the Creeper's explosion. Upon being struck by the player, it will start smoking and flashing white. If there are no blocks to hold up the TNT block, activating it will make it affected by gravity and it will fall. After 4 seconds, it explodes, damaging surrounding blocks, mobs that are nearby, and even the player if they are not careful.

Removing TNT can be tricky, and varies by game. In Survival Test, hitting it two times would stop the countdown and cause the TNT to revert into a resource block which could then be picked up. In Beta, there is no way to pick up a TNT after it has been placed, but it can be detonated without damaging surrounding blocks by immersing it in a liquid, for example by placing a water spring atop the TNT, then detonating it. However this explosion would still damage a player or a mob.

TNT blast will destroy surrounding blocks depending on blocks material. Explosion propagates farthest (loses power the least) through air. This is why an explosion in the open will create the largest destruction radius, but a TNT immediately enclosed in rock will destroy an exact 3x3x3 cube from its center. This feature can be used to confine an explosion differently to make shaped blasts, for example to blast long smooth hallways.

Since the Redstone update, TNT monster traps can now be created and used successfully, making TNT a powerful defense. When exposed to a current, the TNT activates as if it had been activated by a player. Combine this with a Pressure Plate and you can create an automatic defense for your buildings (Although a one-time-use, as the TNT needs to be replaced).

For the TNT to receive the circuit's signal, the redstone needs to be directed straight into it from at least one of four sides, from either the Layer the TNT is based on, or above or below one layer. Diagonally adjacent entry will not make a connection. Finally, Redstone running alongside TNT will also not work due to existing limitations, as it will not branch out into 'T' or 'X' junctions to connect successfully. TNT will only be set off by redstone if the redstone signal it is receiving turns on. This means that if it is receiving signal when it is placed, it will not be set off, but if the signal turns off and turns on again, it will be set off.

Setting TNT on fire activates it after a while. Embers from lava will set off TNT. Water, obsidian and bedrock cannot be destroyed by TNT explosions.

Stationary lava can be destroyed with enough TNT as of version Beta 1.2_02.

Primed TNT
A primed TNT is an entity in Minecraft world. It is spawned at the center (+0.5,+0.5,+0.5) of a destroyed TNT block, while itself is a cube with an edge length of 0.98. Its fuse lasts 80 ticks or a random number between 10 to 30 ticks if it's destroyed by an explosion.

Once spawned, the primed TNT is given a vertical velocity of 0.2 m/s, and a horizontal velocity of 0.02 m/s in a random direction between east and north. This directional bias towards north-east is due to a bug. If it does not collide with another block, it will travel 0.166m horizontally on flat ground before it stops and explodes.

Mining
Mining using TNT might be faster than mining by hand but it is impractical for gathering resources; about 70% of the blocks are completely destroyed in the process. Nevertheless, TNT can be a part of efficient mining techniques - as the means of removing unneeded layers of blocks and exposing underlaying layers. It can also be used for clearing large volumes for underground building projects.

Trivia

 * How to have LOADS of fun using TnT: http://www.youtube.com/watch?v=LJzykDJjBt4
 * Because primed TNT is affected by physics, it is used as a projectile which is propelled by an explosion of a TNT charge in TNT Cannons.
 * Because primed TNT is an entity, objects can pass through it (e.g., water, mobs, players). This feature is also utilized by TNT Cannons.
 * Arrows attached to TNT will fall off when it becomes a primed TNT. This can be used to create arrow traps.
 * A player or mob sitting in a minecart will take minimal damage from a TNT explosion. This can be used to make people/mob cannons.
 * It is possible to 'rocket-jump' via TNT by jumping while standing on it just before it explodes. However, this may hurt, and possibly kill.
 * Sand and Gravel blocks will react accordingly to a detonation, scattering from the point of origin.
 * Sand and Gravel blocks placed on top of a TNT will fall through the TNT when it is triggered.
 * A TNT block can easily be struck fused when hit by a lava ember.
 * The name "TNT" (short for Trinitrotoluene) is technically incorrect. Visually, the block resembles a collection of sticks of dynamite, which is an entirely different explosive frequently confused with TNT. The crafting recipe for the TNT block references the manufacture of dynamite, which is made by stabilizing nitroglycerin with an absorbent material such as sawdust or earth. However, gunpowder is not a component of either explosive.
 * There are some traps in which the player suspends TNT over a 1x1 long pit and activates it. It will drop into the darkness, hopefully killing any unwanted monsters below it. It is especially useful upon entering a dungeon away from the chest to destroy the monster spawn.
 * An activated TNT can be lit on fire in the same manner as arrows can, but it does nothing.
 * Because TNT in a liquid will not destroy any blocks, it is possible to set off TNT safely in the nether, using lava instead of water.
 * Too much TNT too close together might cause lag issues, a way to avoid this is spreading them out a minimum of two blocks or more.

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