Beyond the answers - interesting background to this quiz

The element that refuses to freeze

Helium holds the lowest boiling point of any element, at roughly 4 kelvin, which is why it stays a gas in situations where every other substance has long since turned solid. That extreme behaviour traces back to its tiny, tightly bound atoms and the weak forces between them. It is also the reason helium is used to cool superconducting magnets, including those inside MRI scanners, where no ordinary refrigerant would survive.

A Bond theme with a blockbuster voice

Tina Turner recorded "GoldenEye" for the 1995 James Bond film of the same name, with the song written by Bono and The Edge of U2. It was a fitting match: Turner had already rebuilt her career as a solo superstar in the 1980s, and the Bond franchise has long used its title songs as a showcase for major vocalists. The track reached the top ten in several countries and remains one of the more fondly remembered themes of the Pierce Brosnan era.

Algiers, a capital shaped by the sea

Algiers sits on the Mediterranean coast of North Africa and has been a significant port for centuries, changing hands between empires before becoming the capital of independent Algeria in 1962. Its whitewashed casbah, a dense historic quarter climbing up toward the harbour, is a UNESCO World Heritage Site. The name itself comes from the Arabic word for "the islands", a reference to the small islets that once lay just offshore.

Why 5 stands alone

A prime number has exactly two factors: itself and 1. That rules out 25, which is 5 times 5, and 55, which is 5 times 11. Five is the only single-digit prime that ends in 5, since any larger number ending in 5 is automatically divisible by 5. Primes like this underpin modern cryptography, where huge prime numbers help keep digital communications secure.