There was once a dark cave, deep down in the ground, underneath the earth and hidden away from view. Because it was so deep in the earth, the light had never been there. The cave had never seen light. The word 'light' meant nothing to the cave, who couldn't imagine what 'light' might be.
Then one day the sun sent an invitation to the cave, inviting it to come up and visit.
When the cave came up to visit the sun it was amazed and delighted because the cave had never seen light before, and it was dazzled by the wonder of the experience.
Feeling so grateful to the sun for inviting it to visit, the cave wanted to return the kindness so it invited the sun to come down to visit it sometime, because the sun had never seen darkness.
So the day came and the sun came down and was courteously shown into the cave.
As the sun entered the cave, it looked around with great interest, wondering what 'darkness' would be like. Then it became puzzled and asked the cave, "Where is the darkness???'"
You are the light of the world. A city that is set on an hill cannot be hid. Neither do men light a candle, and put it under a bushel, but on a candlestick; and it gives light unto all that are in the house. Let your light so shine before men, that they may see your good works, and glorify your Father who is in heaven.