Popcorn.gif is a campaign achievement in Halo: Combat Evolved Anniversary. This achievement is obtained when a player kills 100 Flood Infection Forms on the level "Keyes" on Heroic difficulty or above.
A way to do this is to first get the Bandana skull and get to the part where you jump down the hole. If you don't jump Cortana will say "Warning, threat level increasing." and Flood, a lot of them, will come from the small corridor you exited. Throw infinite grenades downrange until you get the achievement!