Beyond the answers - interesting background to this quiz

The Roman numerals that never quite died

MMII is 2002, but the more interesting story is why Roman numerals survive at all. They still mark film copyright dates, monarchs' names and the Super Bowl, which has used them since the first championship game in 1967. The system is additive: each M is 1,000 and each I is 1, so MMII simply stacks up. It is clumsy for arithmetic, which is exactly why Arabic numerals replaced it in European commerce.

The capital that moved twice in one century

Beijing is the capital of China, but it has not always been. Nanjing held the role for much of the early 20th century, and during the Second World War the government relocated to Chongqing. Beijing's name literally means "northern capital", while Nanjing means "southern capital" - a neat reminder that Chinese city names often describe geography rather than history.

Why the iris gets its colour

The iris is the coloured ring around the pupil, and its shade comes from melanin, the same pigment found in skin and hair. Brown eyes contain more melanin; blue eyes contain very little, and their colour is partly an effect of light scattering. The sclera, by contrast, is the white outer layer, and the vitreous body is the clear gel filling the eye.

The Sahel: a belt of transition

Burkina Faso sits in the Sahel, the semi-arid strip that runs across Africa between the Sahara desert to the north and savanna to the south. The name comes from the Arabic word for "coast" or "shore", reflecting how the region was once described as the edge of the desert sea. Sahelian countries share a fragile climate, with unpredictable rainfall and recurring droughts.

Vitamin A and the chemistry of sight

Retinol is the form of vitamin A found in animal foods such as liver, eggs and dairy. The body converts it into retinal, a molecule that reacts to light in the retina and is essential for vision in dim conditions. Severe deficiency can lead to night blindness, which is why vitamin A fortification programmes matter in parts of the world where diets are limited.