Beyond the answers - interesting background to this quiz

The river that crosses borders

The Ganges is one of the most heavily populated river basins on Earth, flowing across northern India before emptying into the Bay of Bengal. It is sacred to Hindus, who regard bathing in its waters as an act of purification, and cities such as Varanasi are built around its banks. Bangladesh, listed in the quiz, shares the delta region through its own great river system, the Padma.

Volcanoes beyond the planets

Most of us picture volcanoes on rocky planets, but the most volcanically active body in the solar system is actually a moon: Io, which orbits Jupiter. Its surface is constantly reshaped by eruptions, and it has almost no impact craters because lava keeps repaving it. The tug-of-war between Jupiter's gravity and the neighbouring moons supplies the heat that drives this activity.

A Nobel gas with a puzzling name

Krypton takes its name from the Greek word for "hidden", a nod to how long it took chemists to isolate it from the air. It sits among the noble gases, which are famously unreactive, and its symbol is Kr - not the K you might expect from potassium. In everyday life, krypton turns up in certain high-performance light bulbs and photographic flash lamps.

A river that runs through six countries

The Mekong begins on the Tibetan Plateau and travels through China, Myanmar, Laos, Thailand, Cambodia and Vietnam before reaching the sea. It supports one of the world's largest inland fisheries, and millions of people depend on it for food and transport. Dams along its course have become a major source of tension between the countries it links.

A language with its own script

Khmer is the official language of Cambodia and the mother tongue of most of its population. It belongs to the Austroasiatic family, which sets it apart from the neighbouring Thai and Vietnamese languages. Written Khmer has one of the longest continuously used alphabets in the region, with roots going back more than a thousand years.