Half-Life Easter Egg - The Alien Filing Cabinet

At the very beginning of the game ,before the accident, after exiting the intro train, type "impulse 101" into the console to get all weapons. Then locate the room where G-man is talking to a scientist. Shoot the glass to get into the room. Go to the small filling cabinet against the wall. You will first observe that the cabinet looks strange when you look at it from different angles, then throw a grenade at the bottom of it and when it explodes alien blood comes out!