"A zener diode is a special kind of diode that acts like a normal diode when it is forward-biased, but also allows current to flow when it is reverse-biased if the voltage difference between the anode and the cathode exceeds a characteristic breakdown voltage. In the water analogy, a diode is like a one-way gate with a spring -- water pressure in one direction will stretch the spring and open the gate, while pressure in the other direction will close the gate. A zener diode is like a gate with a weak spring holding it closed in the forward direction and a stronger spring holding it closed in the reverse direction."