Zener Challenge
To get a feel for how the Zener diode works, set up this circuit on your breadboard:
You can make a 12V supply by wiring two 6V's in series. Any 10k pot will do. You can use a 1N4733A diode, which is a 5.1V Zener.
When you've built the circuit, turn the pot slowly and watch the volt-meter values. Meter A will show the voltage across the Zener, or Vout. Meter B will show the voltage across the pot, or Vin.
Experiment a bit. How does the Zener react to quickly changing voltage levels? What happens when Vin drops below 5.1 Volts?