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 hand grenade in that it is a thrown anti-infantry and anti-vehicle weapon.

Function
The Plasma Grenade sticks to targets and is impossible to release. It also seems to have a slight homing feature similar to that of the plasma pistol. 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. In the most recent update in Halo 2, the Plasma Grenade has an instant kill effect if it goes off at the base of the target's feet (Does not have to be stuck) possibly because Plasma has the ability to melt through metal and would thus cause lethal damage even to a Spartan in MJOLNIR armor.

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, which in one case renders them immune to infection by the Flood.

Theories
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. Since a "dead" wall doesn't emit heat, characters and the heat from vehicles engines causes it to stick. Alternatively the reason a Plasma grenade sticks may also be due to the build up of electrostatic charge as the grenade is thrown through the air. The built up of static will cause the grenade to stick to ungrounded targets such as vehicles and people, but not grounded objects like trees, buildings, or the thrower (while grenade is still in hand).

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.
 * In Halo 3 Beta, it seems that when a plasma grenade is stuck, it emits a high pitch sound before exploding.
 * 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.
 * After you shoot a charged plasma shot from a plasma pistol, when cooling tap the grenade button twice and you will throw two plasma grenades at once.
 * In Halo 3 a panicking Grunt may light 2 plasmas and, whilst holding on to them, run at you kamikaze.
 * In Halo 3 some enemies such as scarabs and high ranking brutes cannot be stuck by these or spike grenades for an unknown reason.
 * In Halo 3, if you look at the grenade bar in the top left corner, you will notice that there is a smilie on the plasma grenade icon, another example of Bungie's humour.