Audio
Some versions of the game have tones that play as long as you push the button down. Others have a constant time of the sound. Other versions feature audio themes: animals (cat/dog/pig/cow), xylophone, football, galaxy (space sounds), some of which (animals, football) make the game easier to play. Yet others can have sound on/off setting, making the game harder by relying just on visual cues.
Read more about this topic: Simon (game)