adding rows exercise - stars exercise - better stars making the fleet move moving the aliens right creating settings for fleet direction checking to see whether an alien has hit the edge dropping the fleet and changing direction exercise - raindrops exercise - steady rain shooting aliens detecting bullet collisions making larger bullets for testing repopulating the fleet speeding up the bullets refactoring update_bullets( exercise - catch ending the game detecting alien-ship collisions responding to alien-ship collisions aliens that reach the bottom of the screen game over identifying when parts of the game should run exercise - game over summary scoring adding the play button creating button class drawing the button to the screen starting the game resetting the game deactivating the play button hiding the mouse cursor exercise - press to play exercise - target practice leveling up modifying the speed settings resetting the speed exercise - challenging target practice scoring displaying the score making scoreboard updating the score as aliens are shot down making sure to score all hits increasing point values rounding the score high scores xx contents in detail