Version 1: UFO moves down, and the cow stays still.
Version 2: The UFO's can be controlled horizontally with keys.
Version 3: The ufo can collide with the cow. The cow is at a fixed,
Version 4: Derived from version 1. The cow moves from right to left,
and the ufo can move straight down.
Version 5: Added keyboard control back to the ufo as well as collision
against the cow.
Version 6: There are now multiple cows.
Version 7: The cows can now also move from left to right as well as
right to left.
Version 8: Cows can collide with each other or edges of the scene, in
which case they change direction.