Software version control systems range in features and difficulty. Git is one of the more feature rich, which is why it’s used for developing Linux itself, but it can be a bit intimidating. This is a good “start here” page.
CdS Cells, Photoresistors, & Light Dependent Resistors (LDR) Source: Using a Photocell | Photocells | Adafruit Learning System
Make your Arduino walk and chew gum at the same time. Source: Overview | Multi-tasking the Arduino – Part 1 | Adafruit Learning System