In Alien Invasion, the player controls a ship that appears at the
bottom center of the screen. The player can mve the ship right and left 
using the arrow keys and shoot bullets using the spacebar.
When the game begins, a fleet of aliens fills the sky and move across and down 
the screen. The player shoots and destroys the aliens. If the player shoots all the 
aliens, a new fleet appears that moves faster than the previous fleet.
If any alien hits the player's ship or reaches the bottom of the screen, the 
player loses a ship. If the player loses three ships, the game ends.