Beyond the answers - interesting background to this quiz

The planet that shrinks as it ages

Mercury is the smallest planet in the solar system and the closest to the Sun, but its most striking trait is its composition: it is unusually dense and rich in iron, with a metallic core that makes up a huge share of its volume. Because the planet has cooled and contracted over billions of years, its surface is scarred with long ridges and cliffs known as lobate scarps. In some ways it resembles a metal-rich world more than a rocky one.

Bronze, the alloy that named an age

Bronze is roughly copper with tin added, and the classic mix sits close to 88% copper and 12% tin. The discovery that tin could harden copper was a genuine technological leap, producing tools and weapons that held an edge far better than pure copper. Bronze became so central to early metallurgy that an entire period of human prehistory, the Bronze Age, is named after it. Different cultures stumbled onto their own recipes, sometimes using arsenic instead of tin.

Two capitals that are not the biggest city

Hanoi is the capital of Vietnam, while Ho Chi Minh City, formerly Saigon, is the country's largest urban centre and economic engine. The same split shows up in India, where New Delhi serves as the capital while Mumbai dominates finance and Bollywood. Karachi, meanwhile, is a major Pakistani port city, not an Indian one. Capital-versus-largest-city confusion is one of the most common traps in geography quizzes.

Copenhagen and its Baltic neighbours

Copenhagen is the capital of Denmark, a country whose coastline touches the Baltic Sea along with the Øresund strait. Finland also sits on the Baltic, sharing it with Sweden, Poland, Germany and the Baltic states. The sea is brackish rather than fully salty, a quirk that shapes its unusual mix of marine and freshwater species. Helsinki and Riga, the other options in this question, are the capitals of Finland and Latvia respectively.

Fluorine, the element that wants electrons most

Electronegativity measures how strongly an atom pulls shared electrons toward itself in a bond, and fluorine tops the scale. Sitting at the far corner of the periodic table, it is the most reactive of all elements and rarely appears in nature on its own. That hunger for electrons is why fluorine forms such stable compounds and why it turns up in everything from non-stick coatings to toothpaste.