Because they have an infinite number of ways to self reflect
Pheidippides (530 BC-490 BC), an Athenian herald, was sent to Sparta to request help when the Persians landed at Marathon, Greece. He ran 240 km (150 miles) in two days. He then ran the 40 km (25 miles) from the battlefield by the town of Marathon to Athens to announce the Greek victory over Persia in the Battle of Marathon (490 BC) with the word "Νενικήκαμεν" (Nenikékamen, 'We have won') - and died on the spot.
This depends. A "gram" is a measure of mass, while a "cc" is a measure of volume, so this can't really be answered. The two measure different things and basic dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.
As a simple mental exercise, consider a cm3 of air and of water. They will have very different masses.
It is the average price level of all the goods and services in the country.