For thousands of years the major civilizations of the world thrived in isolation from one another. These civilizations had very little contact with one another, and in many cases had no knowledge of each other.
This all began to change during the Dark Ages. Both Western Europe and China began to send out explorers in an effort to learn about the world around them. These explorers helped to expand the knowledge of their people.
As a result of their explorations, trade routes opened up between civilizations. Each nation specialized in certain types of products and goods. These goods would be shipped to far away nations in exchange for the goods that they specialized in.
Sometimes the best helping hand you can get is a good, firm push.
Seven of the eight US Presidents who have died in office - either through illness or assassination - were elected at precisely 20-year intervals.