Spotter | Adding some pop.

The core mechanics of the game are now complete, but we can extend the development to embed what we have learned and create a better game. These extensions are presented in three levels of increasing difficulty we call, 'snap', 'crackle' and 'pop'. You are now on your own, but challenge yourself to see how far you can get using the stages in the tutorials for reference and other online sources.

1. Limit the number of mistakes the player can make by introducing a time penalty every time a cross is placed incorrectly. This impacts the value of the countdown timer.
2. The game should display a "game over" message before returning to the title screen when the time runs out.
3. Introduce a score so that when the correct number of differences have been found for the level it automatically progresses to the next level.