Beyond the answers - interesting background to this quiz

A conflict measured in decades, not battles

The Cold War is usually dated from the end of the Second World War to the early 1990s, which places it firmly in the 20th century. It was called "cold" because the United States and the Soviet Union never fought each other directly, instead competing through proxy wars, espionage and an arms race. Its most famous symbols, from the Berlin Wall to the space race, all belong to that same crowded century.

The electron, the first piece of the atomic puzzle

J.J. Thomson identified the electron in 1897 while studying cathode rays at Cambridge. His discovery showed that atoms were not indivisible after all, but contained smaller particles, a finding that reshaped physics. Thomson won the Nobel Prize in Physics in 1906, and his son George later won the same prize for work on how electrons behave inside atoms.

Paper, an invention that travelled the world

Paper is traditionally credited to China, where it was being made by the early second century. From there the technique spread slowly along trade routes into the Islamic world and eventually into Europe, where it gradually replaced parchment and papyrus. It is hard to think of a single invention that has carried more ideas, laws and stories across the centuries.

Mountains that shape a continent

The Himalayas lie in Asia and form the highest mountain range on Earth, home to Mount Everest. They were raised by the slow collision of the Indian and Eurasian tectonic plates, a process that is still pushing the peaks upward by a few millimetres each year. The range also feeds some of the great rivers of Asia, including the Ganges and the Brahmaputra.

The long border between the United States and Mexico

Mexico shares its northern border with the United States, a line stretching roughly 3,000 kilometres from the Pacific to the Gulf of Mexico. It is one of the busiest land borders in the world, crossing deserts, rivers and major cities such as Tijuana and El Paso. The other countries sometimes confused with it, Brazil and Peru, lie far to the south in South America.