My love, when I gaze on thy beautiful face,
Careering along, yet always in place
The thought has often come into my mind
If I ever shall see thy glorious behind.

What am I talking about?

To protect his country a king decided to build a wall made out of stone along the border. The wall had to be 3 meters thick and 5 meters high. The border of the country has a length of 500 kilometers. Assuming that one cubic meter of stone weights about 3000 kg, how much heavier becomes the earth by building this wall?

Who makes it, has no need of it.
Who buys it, has no use for it.
Who uses it can neither see nor feel it.