Beyond the answers - interesting background to this quiz

The family that hardly reacts

Helium, xenon and their relatives occupy the far right column of the periodic table, a group known as the noble gases. What unites them is a full outer shell of electrons, which makes them unusually reluctant to bond with other elements. For decades chemists assumed they were completely inert, until xenon compounds were successfully produced in the early 1960s. Oxygen, by contrast, sits elsewhere on the table and reacts with almost everything, which is precisely why it keeps us alive.

A lake with no outlet to the sea

The Great Salt Lake in Utah is a remnant of Lake Bonneville, a vast prehistoric body of water that once covered much of the region. Because the lake has no outlet, water can only leave by evaporation, leaving dissolved minerals behind. That is why its salinity is far higher than that of the ocean, and why swimmers find it remarkably easy to float. Only a few hardy organisms, such as brine shrimp, thrive in its waters.

Bangkok's very long official name

Most of the world calls Thailand's capital Bangkok, but Thais themselves usually say Krung Thep. The full ceremonial name is one of the longest place names in the world, a mouthful that begins "Krung Thep Mahanakhon" and continues through a string of auspicious epithets. The city sits on the Chao Phraya River and has long served as the country's political, cultural and economic heart.

A film that rewrote the rules

Orson Welles directed, co-wrote and starred in Citizen Kane, released in 1941 when he was still in his mid-twenties. The film is celebrated for its deep-focus photography, its fragmented narrative structure and its playful use of light and shadow. It centres on a newspaper magnate whose dying word, "Rosebud", sends a reporter digging through his past. Critics have ranked it among the greatest films ever made for decades.

Roman numerals and the art of subtraction

Roman numerals work largely by addition, but they also use a subtractive principle to keep numbers short. A smaller symbol placed before a larger one is taken away rather than added, which is why 17 is written as XVII. The system has no symbol for zero and no straightforward way to express very large numbers, which is one reason it was gradually replaced by the numerals we use today.