Answer: Mongols did not necessarily need to expand their territories as much as they did. The pull factor for Mongol imperialism is directly related to the push factors. They may have included climactic and geographic issues impacting the nomadic tribes but most likely were due to the Mongols being enthusiastically militaristic.
Answer: retrograde motion
Retrograde motion is an apparent change in the movement of the planet through the sky. It isn't real in that the planet doesn't physically start moving backwards in its orbit. It just appears to do so because of the relative positions of the planet and Earth and how they're moving around the Sun.
A quickly again population: China is going to be the first major economy to go dull before if gets rich, putting huge strains on people in the working age who are going to have to pay for health care and pensions for a disproportionate number of elderly citizens who do not work. For a nominally socialist county, the weave for china's social safety is very loose and horribly frayed in some parts. Most notably their healthcare system. This can be often cited as a major cause of the pretty high savings rate. Something that often chockes off consumption and adds to the imbalance in China's economy.
