The Plasma Grenade (or sticky as it's sometimes referred to) is a specially designed grenade that adheres to its targets. Once it has made contact with a target, the plasma grenade will immediately bond to it, 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 armor or a weapon, removing said articles will allow one to distance themselves from the explosion. The plasma grenade once it has adhered to the target it kills instantly, so all there is to do is kiss your butt goodbye if your the one stuck.
The "sticking" mechanism is complex enough to allow the plasma grenade the ability to distinguish between targets and inanimate objects. 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.
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 also known as kamikaze. In addition, the plasma grenade can be destroyed by being shot (Easily seen with the Sentinel Beam).
Also, some characters in campaign cannot be stuck. Brute Chieftains have immunity from this until their armor is knocked off. However if the Brutes shield is broken (i.e in Halo 2, Tartarus can be stuck if his shield is broken), you can still stick them.
Sticking an enemy with the plasma grenade, will always result in instant death (except for hunters).Also, except when (in Oddball, or Juggernaut) the player is stuck and the option "Ballcarrier has extra life" is chosen (in Halo: Combat Evolved), but in this situation the ball carrier will only have 1 bar of health. Even a fully shielded Spartan or Elite (except on higher difficulty levels) cannot withstand a stuck plasma grenade. In Halo 3, high ranking Brutes on the harder difficulties can withstand a player's grenade with Mythic (death skull). Also, higher ranking Grunts on Easy, can withstand grenades thrown by their Grunt brethren if it sticks to them with Mythic. In Halo: CE Grunts will sometimes run for surrounding elites when stuck, pleading him to remove it. This results in both the grunt, the elite and the elite's team, if any, being killed.
The plasma grenade's bright blue glow and longer delay fuse allows enemies to easily notice and avoid it. It cannot be used around corners, as its bounce is negligible, and it does not roll. A player wearing a fully charged Overshield may be capable of surviving the blast even when stuck. Also, if a smart enemy is stuck when they are too close they can simply run toward you to catch you in the blast zone. In Halo 2 and Halo 3, a primed plasma grenade in flight can be detonated mid-air by another explosion. Weapons capable of this include the Rocket Launcher, Brute Shot, Missile Pod, Frag Grenades, other Plasma Grenades. The Plasma Grenade will not stick to certain types of armor such as that of a Scarab or a Brute Chieftain's. When thrown at them, the grenade will fall to their feet, and they will probably notice it and move out of the blast radius. Ballistic or explosive weapons such as a shotgun or Brute shot will remove the plating allowing the grenade to stick. If someone throws a plasma and it hits the wall, it can bounce off the wall back down and adhere to the thrower. Also, if the opponent notices you, he/she will probably take immediate evasive action. As you will probably miss if they dodge, they will be relatively unaffected due to the tiny blast radius.In addition, due to the long explosion time, if you intend to flip a vehicle by planting a grenade, the driver will have plenty of time to swerve and dodge. This also applies for infantry, as Grunts will dive out of the way if not stuck.
Theory of "Stickiness"
Note: The Following is taken from Halopedia
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 is most likely what causes it to stick. 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.
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 interesting thing, to note 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, 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 proved by the fact that it will stick the person who threw it if it bounces back at them i.e a wall.
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 as made sticky as glue by the activation of the adhesive. Although, this theory doesn't explain the environment/character principles of the grenade.
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 armor.
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 MJOLINR Armor, Elite Combat Harness, and Brute Combat Harness all are damaged by projectiles. This could mean that the Plasma grenade cannot stick to a high level electrical pulses, such as the ones that are in Jackal Gauntlets, but have 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 armor has extremely powerful shielding, albeit it can still be damaged by human projectiles.