Beyond the answers - interesting background to this quiz

The element that hides in plain sight

Oxygen is usually thought of as the gas we breathe, but it is also the most abundant element in the Earth's crust, making up roughly 46 percent of it by weight. Most of that oxygen is not floating free - it is locked inside minerals and rocks, chiefly in compounds with silicon. Silicon itself comes second, at around 28 percent, which is why the pair dominate the rocks that form continents.

A planet with a crowded sky

Jupiter holds the record for confirmed moons, with the count passing 90 in recent years thanks to surveys that keep finding small, faint objects. Saturn is not far behind and has often traded the lead as new discoveries are announced, so the title is best described as provisional. Either way, both gas giants have far more known satellites than the inner planets combined.

Ali's career beyond the ring

Muhammad Ali won the world heavyweight championship three separate times and became one of the most recognisable athletes of the 20th century. His refusal to be drafted into the U.S. army during the Vietnam War cost him his titles and licence for several years, and the legal battle that followed reached the Supreme Court. Later in life he became a global figure for humanitarian work, and his public struggle with Parkinson's disease raised awareness of the condition.

Polo, a sport measured in furlongs

Polo is played by two teams of four riders, each wielding a long mallet to drive a small ball between goalposts. A match is divided into periods called chukkas, usually six or eight of them, and each chukka lasts about seven minutes. The field itself is enormous - around 270 by 150 metres, roughly the size of three football pitches side by side.