Type-1 Antipersonnel grenade

The Type-1 Antipersonnel Grenade is a grenade designed by the Forerunners and used by the Covenant infantry. The grenade sticks to its target if thrown accurately, and detonates automatically in three seconds. It can also be manually detonated by a button press or by firing onto it. Few technical details are known about the Plasma Grenades, since they are of alien origin. Engraved on the Plasma Grenade is a Forerunner symbol, though its appearance was inconsistent, being changed in every game within the Halo trilogy.

Blue Gas
After a Plasma Grenade is ignited and thrown, it begins to leak a densely small and bright cloud of plasma blue gas, believed to be between 60 to 75 degrees. This occurs due to the explosive reaction taking place inside.

This cloud will gradually grow to a larger diameter after each second of the countdown, and also causes a trailing effect, as seen in all games.

Stickiness
The Plasma Grenade is a specially designed grenade that sticks to its targets. Once it hits a biological creature or a vehicle, the Plasma Grenade will immediately bond to it, and beep to tell you that it has stuck, making any removal impossible. The bonding mechanism holds its attraction, except to seemingly static objects, such as walls, until detonation, making it impossible to avoid its deadly explosion. Therefore, if you have been "stuck", it is a good idea to take out as many opponents around you to reclaim points. Although it will bond to any target it hits, if it attaches to a piece of armour or a weapon, removing said articles will allow one to distance themselves from the explosion. Once the Plasma Grenade has adhered to the target, it kills instantly. All one can do is to try and get as close to an enemy as possible, with the hope of killing them or at least damage them, thus leaving them vulnerable to any attack from another hostile. Although the Plasma Grenades can stick to the enemies or friendlies, in Halo: Combat Evolved, it can be seen clearly that the Frag Grenade is much more powerful, but due to the enemy or friendly being stuck the explosion is close range thus the enemy being stuck has a higher chance of dying. It can easily be identified by its blue glow.

The "sticking" mechanism is complex enough to allow the Plasma Grenade the ability to distinguish between targets and inanimate objects (the technology behind this is unknown, although it is thought to be vaguely Forerunner). For example, it will stick to a soldier or a vehicle, but not a tree or a wall etc. This does not, however, allow the grenade to act selectively. The Plasma Grenade will not distinguish between friend or foe, so accuracy and caution is strongly advised when throwing it, so as not to betray your allies.

Theories

 * Note: The following is speculation and theory.

Sticking to Targets
The Plasma Grenade may stick by producing a thin layer of plasma which, when in contact with a target, fuses to the target. The reason the Plasma Grenade sticks to vehicles and characters may be due to an ability to sense body heat and the heat from vehicles' engines. Since a "dead" wall doesn't emit heat, it will, however, stick to the "living" walls of a Flood Hive. One problem with this theory is that it will stick even to exposed skin (on Grunts, Jackals, or Marines as an example) even though there is no metal to fuse to - it would be nearly impossible to fuse the grenade to flesh using heat alone.

Another theory is that when the grenade is thrown it will be covered by a thick layer of extremely hot plasma that when stuck to anything, will burn into the object, sticking there, and then explode.

It is also theorized that there is a complex internal mechanism built into the grenade that gives it this characteristic. A gravity generator may be built into the core of the device, giving it its own force of attraction. This artificial force of attraction is highly concentrated, which is exactly why the grenade is impossible to remove.

Another theory is that the Plasma Grenade has a energy shield hybrid shell to bounce off of walls and an internal magnet to pull it to vehicles and armour.

Yet another theory states that the Plasma Grenade detects low-level and medium level electrical pulses that are in living things and that are in shield and vehicle technology. Jackal Gauntlet Defense systems emit a high rate of energy, enough to repel Human Projectiles, while the MJOLNIR Armour, Elite Combat Harness, and Brute Combat Harness all are damaged by projectiles. This could mean that the Plasma Grenade cannot stick to a device emitting high level electrical pulses, such as the ones that are in Jackal Gauntlets, but has sufficient energy to stick to low to medium power energy shields as well as vehicles. This would also explain why Brute Chieftains cannot be stuck, as their armour has extremely powerful energy shielding, albeit the fact that it can still be damaged by human projectiles.

Another theory is that the grenade has a small A.I. that can allow it to understand what and what not to stick to. This would allow it to stick to any kind of flesh material and also let it to stick to the walls of the level Cortana. This would also allow it to stick to vehicles.

Sticking to Throwers
An interesting thing to note is the fact that when a Plasma Grenade is primed, it does not stick to the thrower. A theory for this could be that the grenade must be primed touching something emitting heat, and then leaving the heat-emitting object, thus activating its "stickiness", and finally landing and sticking to another heat-emitting object (i.e. the target). This is even better proven by the fact that it will stick to the person who threw it if it bounces back at them (such as hitting a wall). This theory is also supported by the fact that Kamikaze Grunts can hold them indefinitely, although it seems the Grunt has control over when it explodes.

It's possible that the grenade has an adhesive activated by a timer. Once the grenade is activated, this timer is initiated, giving a few seconds for the thrower to release the grenade. Once the timer has run out, the adhesive activates and the surface of the grenade is made as sticky as glue by the activation of the adhesive, although this theory doesn't explain the environment/character principles of the grenade.

It is also possible that it will not stick to the thrower because the user may have to hold down the activation switch and upon releasing the switch it will become sticky.

Detonating Timer
The Plasma Grenade possesses a timer. It has a three-second fuse that counts down at the point of activation. A manual activation via a small button or a stimulated activation, such as another explosion or gunfire, will set the detonator and start the countdown. Delayed activation can also set the detonator after it either sticks to a target or otherwise comes to a rest. It is possible to tell if the grenade has been activated. A densely small and bright cloud of plasma emission will leak around the grenade as the explosive reaction takes place inside. This cloud will gradually grow to a larger diameter after each second of the countdown. A low sound is emitted by the grenade when activated, followed by two rapid beeps and a high-pitched whine just seconds before the explosion. This is the Plasma Grenade's standard process of detonation.

The Plasma Grenade appears to have two settings: the standard three-second fuse and a delayed manual detonation. The latter is seen in Halo 3 by suicidal Grunts hoping to ensure their enemy's death by exploding right next to them. In addition, the Plasma Grenade can be destroyed by being shot (easily seen with the Sentinel Beam).

Effects on Health
It has been shown that prolonged exposure to the radiation emitted by Plasma Grenades, without proper protection, can cause a deviation in the neural electric pathways of the user, a disorder called Boren's Syndrome. This disease can be fatal if the person does not get treatment. Sergeant Avery Johnson is the only known living person to have Boren's Syndrome. This explains why he was immune to infection by the Flood during the attack on his team on Installation 04 while trying to extract a weapons cache. The reason for this is because the Flood cannot copy the organisms nerve endings because they are "messed up" (as said by Master Chief in Halo: First Strike), so the host never gets infected, although it can still be killed by one because of attack. Boren's Syndrome is not simply immunity to the Flood, it is known to cause mental instability as well as other things, making it impossible as a weapon of self defense against the Flood (except in the case of Staff Sergeant Avery Johnson who, for unknown reasons, seems to be immune to the side-effects, even though he rejected the treatment and cure).

Strategies
The Plasma Grenade releases a very powerful and intense explosion, that when stuck to a Sangheili or Spartan, its blast completely drains their Energy Shields, cutting cleanly through their defenses and killing them instantly.

Basic

 * The Plasma Grenades from Halo: CE are probably meant for taking out masses of targets based on its rather large blast radius. In subsequent games its blast radius is reduced.
 * According to the Halo 3 game manual, the Plasma Grenade has the second highest kill and casualty radius (four meters and twelve meters, respectively) of all the grenades in the game.
 * In Halo: CE, Plasma Grenade damage, like a Needler detonation and plasma pistol overcharge, is classified as an EMP (electromagnetic pulse) and instantly drops all shields, even a full Overshield. The same held for grenades that were dropped. This was removed in Halo 2 and Halo 3 (except for the plasma pistol overcharge), making it less effective against infantry.
 * Like all grenades, it can be shot and detonated by a weapon. A Battle Rifle or Sniper Rifle work best.
 * If a player's weapon is stuck with a Plasma Grenade, it is possible to pick up another weapon, thereby dropping the "stuck" weapon, and live. The Plasma Grenade will stay on the dropped weapon and still explode, though not be considered a stick. Switching for your secondary weapon won't help, since you still carry the "stuck" weapon.

Advanced

 * It has actually been known for players to somehow "remove the sticky" and throw it back, but on closer inspection you will notice that it only works if you stick your hand, then throw a grenade or a piece of equipment.
 * Occasionally, if two Plasma Grenades are thrown in succession at the same spot; one will detonate, launching the other far into the air. The airborne grenade will explode in mid-air. This is easiest to perform in Halo: CE. A similar effect occurs when they hit each other while in mid air, causing them to bounce off each other. This is best seen in the Top 10 Halo 3 Suicides video.
 * If the user can throw the Plasma Grenade onto an Equipment and it rolls inside a Bubble Shield the Plasma Grenade will stay on the Piece of Equipment.
 * In a Forge game, after getting stuck as a Sangheili/Spartan, turning into monitor mode will cause the grenade to not detonate. But if you turn into Sangheili/Spartan an the grenade will explode.
 * In Halo 3 if you throw a Plasma Grenade at the right spot on the Pelicans cockpit it will stick the driver, kill him/her and the Pelican will fall below the map.
 * In Halo 2 if you stick an Elite Ranger, they will be unable to escape the explosion, even though equipped with zero-gravity backpacks. If a grenade is thrown on nearby ground, they will simply fly to a distance, escaping unscathed.
 * In Halo 2, the Heretic Leader Sesa 'Refumee and first main antagonist in the game, can be easily killed by sticking him with a single Plasma Grenade. The Arbiter's Active Camouflage must be used in conjunction, in order to get close enough to correctly calculate the trajectory of a Plasma Grenade.
 * You can actually suicide bomb with it. If you use the trick above in Forge so you don't explode, you can fly right up to a person and the change back into an Sangheili\ Spartan, you will explode right by them.
 * In Halo 3, if the Plasma Grenade is meleed while its laying on the ground inactive, there is a 75% chance that it will detonate since the action requires applying pressure to the button that activates the grenade.

Multiplayer

 * As a stuck player will be killed even if possessing overshields, Plasma Grenades are more commonly used than frag grenades.
 * They are often used as a last ditch effort. When about to be shot down, many will stick their opponent effectively killing them "from the grave".
 * In Halo: CE, infinite grenades may be applied to a game style. This is entertaining but results in tremendous lag on online play.
 * In gametypes such as Capture the Flag, and Neutral Bomb many defending players would utilize the Plasma Grenades to either destroy incoming vehicles from the opposing team, or kill any escaping enemies.
 * In an assualt or CTF game it is a good idea to get two of these grenades and stick on coming veichles or groups of players. This works well on Standoff, Avalanche, and Sandtrap.
 * In Halo 3, the plasma grenade can be slammed away by a gravity hammer, but making the plasma grenade in one second explode from the force of the hammer, making this tactic risky.

Failures

 * Sticking a vehicle will not always destroy it and the passengers inside. Many "sticks" have been documented in which the vehicle was stuck but the passengers were not killed, merely hurt.
 * Sticking a mongoose will kill the the driver and passenger.
 * Some units cannot be stuck, such as Brute Chieftains, who have immunity from sticking with their armor being designed to combat the effects of Plasma Grenades. However, their weapon can still be stuck, which results in an immediate kill as usual.
 * Some units, such as Hunters, can be stuck and will get damaged by a grenade explosion, although a single grenade will fail to kill them. This is due to their immense physical strength and tough rhino-like skin which substitutes for Energy Shields.

Target Reactions

 * If a Grunt is stuck, they will usually run into their comrades trying to get it off, but if you stick an Elite or a Brute they will either charge you or take cover in an attempt to take you out.
 * In Halo: CE, Grunts may occasionally yell "Not again!", which could mean they could have survived an explosion from a Plasma Grenade before, probably by jumping away in the nick of time.
 * In Halo 2, when the Covenant use Plasma Grenades in battle, they will shout phrases as they throw them, such as "cleansing flame", "holy flare" and "holy light". On rare occasions, they will still shout "Demon flare!" even if it is thrown by an ally. These quotes may correlate to the meaning of the Forerunner symbol(s) printed on the Grenade.

Internal

 * Type-3 Antipersonnel/Antimatériel Incendiary Grenade
 * Type-2 Antipersonnel Fragmentation Grenade
 * M9 High-Explosive Dual-Purpose Grenade