Anskum-pattern plasma grenade

The Covenant Type-1 Antipersonnel Grenade, also known as the Plasma Grenade or by fans as Sticky Grenade, is similar to the UNSC Frag Grenade in that it is a thrown anti-infantry and anti-vehicle weapon.

Overview
The Plasma Grenade sticks to targets and is impossible to release. It has some kind of internal mechanism that allows it to distinguish between targets and inanimate objects. For example, it will stick to a soldier or vehicle, but not a tree or wall. It has a three-second fuse that is activated after it sticks to a target or otherwise comes to rest. If it successfully sticks to an enemy soldier or vehicle, the grenade will rapidly beep twice then emit a high pitched whine shortly before exploding. The Plasma Grenade appears to have two settings; the standard three second fuse and a manual detonation. The latter is seen in Halo 3 by suicidal Grunts hoping to ensure their enemie's death by exploding right next to them.

Advantages
Sticking an enemy with the plasma grenade kills them when it detonates. Even a fully shielded Spartan or Elite (with the exception of higher-ranking Elites like Ultras on higher difficulty levels) cannot withstand a stuck plasma grenade (it even kills if they have overshields). Also, the plasma grenade is slightly more powerful than the UNSC fragmentation grenade. If it goes off at the base of the target's feet (Does not have to be stuck) it results in an instant kill. This attribute is only featured in Halo 2 and in Halo 3. The one exception in Halo 2 is if a player has a fully loaded Overshield the grenade will heavily damage him and can be killed almost immediately after. In Halo 3, the Overshield is weaker than in Halo 2, so a stick is lethal in Halo 3 regardless of shield status.

Disadvantages
The plasma grenades bright blue glow and longer delay fuse allows enemies to easily avoid it. It can not be used around corners well, as it does not bounce, nor does it roll. Also, if a smart enemy is stuck when they are too close they can simply run toward you to catch you in the detonation. 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 enemies such as Scarabs and Brute Chieftains. It should be noted that prolonged exposure to the radiation emitted by plasma grenades 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.

Stickiness Theory
Note: the following is speculation and is not considered canon.

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 heat. 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.

Trivia

 * In Multiplayer, if you stick an enemy with the plasma grenade you are awarded with the Grenade Stick Medal.
 * In Halo 2, when the Covenant use plasma grenades in battle, they will shout phrases as they throw them, such as "cleansing flame", "flare" and "holy light". This is similar to their belief of Halo.
 * If John-117 throws a plasma grenade, the Covenant will call it a "demon flare". However, when the Arbiter throws a plasma grenade, Elites and Grunts will still address it as a demon flare, even if the grenade is a frag. This may prove that all kinds of enemy grenades are demon flares. This may be as a kind of slang, with "demon" referring to the enemy, or enemy thrown grenades, and holy in the opposite manner.
 * Often times, when an NPC is stuck, they will have some type of reaction to being stuck.
 * Occasionally, if two plasma grenades are thrown in succession at the same spot; one will detonate, launching the other into the air. The airborne grenade will explode in mid-air, causing a "fireworks" effect. This was easier to perform in Halo: Combat Evolved.
 * If you stick a charging Hunter with a plasma grenade the Hunter will instantly turn around, exposing its back, when the grenade explodes.