Place a lightbulb in a glass of water so the metal base is submersed and place in the Microwave. Turn on. The light bulb won't turn on. Remove the glass and place salt in the water. Place in microwave again and turn on. This time the bulb will light up.

Analogies:

Our good example makes it possible for the light of christ to shine throughout the earth.
Light - Light of Christ
Salt - People are the salt of the earth ~ our good example or our faith.
Microwave - Earth

OR

Miracles happen by faith.
Salt = faith
Light = Miracle.