Beyond the answers - interesting background to this quiz

A capital named after a wind

Nouakchott, the answer to the Mauritania question, is one of the youngest capitals in Africa. It was built largely from scratch after the country gained independence from France in 1960, growing from a small fishing village into a city of well over a million people. Its name is often linked to Berber words for "place of the winds", a fitting label for a city on the edge of the Sahara. Dakar and Bamako, the decoys, are the capitals of Senegal and Mali respectively.

The world's most spoken mother tongue

Mandarin Chinese tops the list of languages by native speakers, far ahead of Spanish and English. That ranking changes completely when you count people who speak a language as a second tongue: English then moves to the front, largely thanks to its role in business, science and the internet. The two measures answer two different questions, which is why headline rankings often seem to contradict each other.

The Amazon does not stay in Brazil

Most of the Amazon basin lies in Brazil, but the river and its tributaries also cross Peru, Colombia and Bolivia, the country named in this quiz. Bolivia's share includes stretches of the Madeira and Mamore systems that feed the main river. The Amazon carries more water than any other river on Earth, and during the wet season its flow into the Atlantic is enormous.

Carbon, the raw material of plant growth

Plants take in carbon dioxide through tiny pores in their leaves and combine it with water to build sugars, releasing oxygen as a by-product. This process, photosynthesis, is the foundation of nearly every food chain on the planet. The carbon locked into plants also returns to the soil and atmosphere, which is why forests matter so much in discussions about climate.

Numbers with a pattern and a past

The series 1, 8, 27, 64 is built from cubes: 1x1x1, 2x2x2, 3x3x3 and so on, making 125 the next term. Roman numerals follow a different logic altogether, using letters rather than place values. XXV is simply 10 plus 10 plus 5, a reminder that the system has no symbol for zero and relies on addition and subtraction rules that still appear on clock faces and monuments today.