From Halopedia, the Halo wiki
The Grenade Stick (also called Stick It in Halo 2) medal is awarded for killing an opponent by sticking a Plasma or Spike Grenade to them. It is available in all multiplayer gametypes and has a Bungie difficulty rating of 3 out of 10. It is represented by a 16-pointed bronze star with a 16-pointed blue star in it, and a blue sphere inside that. After you kill an opponent by sticking a grenade to them, a message appears on your screen that says: "You stuck (player's name)". This is also a medal in the Halo 3 campaign. Sticking is very easy to do if you have the Catch Skull, because when an enemy leans to throw a grenade at you, they cannot dodge any grenades thrown at them during this time.
It has also been seen as a rare occurrence that players can also be "stuck" with a frag grenade, which appears to get stuck under the helmet. This has happened as a glitch both at random times, and when players attempt to throw a frag grenade before completely exiting a bubble shield. Even when "stuck" with a frag grenade, the medal is still awarded and counted as a "stick". This is because of the game's scripting that says that if a grenade (besides a firebomb) is touching you when it explodes, it counts as a "stick".