After a long way, a truck driver ends up in front of a tunnel. He can not get through without severely damaging his truck, because the tunnel has a height of 3.00 meters, while his truck has a height of 3.01 meters. The driver would like to pass the tunnel because the other way will take him at least 1 hours. Suddenly he gets a great idea, he immediately starts doing some things and after 10 minutes he is back on track on the other side of the tunnel. What was the great idea of the truck driver?

What has four legs and only one foot?

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?