The Shrink Ray is a weapon that shrinks things like cars, crates, boxes, containers, and humans. You can't shrink buildings, boss level characters, Blasto, yourself, and the game floor. Also, you can't PK anything that got shrunk. Once a human or enemy get shrunk they will not attack you, the humans will run away in terror and the tanks will just stop what they are doing. It's only obtainable after completing the Destroy All Humans! Big Willy Unleashed and selecting it from the Cheat Menu.