If you are in a happy and silly mood try this: at the first level when the captain gives you a pistol, get out of the room to the place where you see the first grunts. kill them and go back to the captain's room and kill him. Its so funny. Cortana will say, " WHAT THE HELL ARE YOU DOING? Security to the bridge. The master chief has gone crazy". Then you will get locked inside and the troops get invincible and come and attack you because they think you are a traitor. (if you are playing 2 player mode and one of you kills the captain, the other player poofs to the captain's room and gets locked too)