This article for the young and old talks about the origins of our number system and the important role zero has to play in it.
A Romantic Riddle
Age 7 to 11 Challenge Level
I ask you, sir, to plant a grove
To show that I'm your lady love.
This grove, though small, must be composed
Of twenty-five trees in twelve straight rows.
In each row five trees you must place
Or you shall never see my face.