In Space Invaders Part 2, from level 2 on, a UFO will periodically fly across the top of the screen and replace invaders that you have already killed. While this is irritating, you can put the new aliens to good use. If you shoot an alien as it is being dropped from the UFO, but before it takes its place in the row, it will fall to the ground dead and act as another fort! It will absorb about 3-5 hits and can really save you on later levels.