10 most difficult ma­te­ri­als riddles

Auxetics are structures or materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their particular internal structure and the way this deforms when the sample is uniaxially loaded. Auxetics can be single molecules, crystals, or a particular structure of macroscopic matter.

Angora goats have been in Central Asia since the Paleolithic. They produce the luxury fibre known as mohair. It is lustrous and shiny, warm in winter as it has excellent insulating properties, while remaining cool in summer due to its moisture wicking properties. Today, South Africa is the world's largest mohair producer, producing around 50% of the total world production.

Common usage defines the fibre as a wool, but in fact it is a hair, and this is what gives it its unique characteristics as compared to sheep's wool (a hair has a hollow core, while wool does not). Cashmere has been manufactured in Nepal and Kashmir for thousands of years.

Damask was first produced in China. Its use spread over India, Persia and Syria on the Silk Road to Europe. Damasks derive their name from the city of Damascus-in that period a large city active both in trading (as part of the silk road) and in manufacture. Damasks became scarce after the 9th century outside Islamic Spain, but were revived in some places in the 13th century.

The Pima cotton strain is widely revered to be the finest of all cottons. Pima cotton originated in Peru, but earned its name from the Pima tribe of American Indians who pioneered the cultivation of this superior strain of cotton in the south-west United States. Today, Pima is grown in Peru, Australia, and the United States. Pima is also the strongest of all cottons, making fabrics that are durable and long-lasting.

Cotton was independently domesticated in the Old and New Worlds. Fragments of cotton fabric dated from 5000 BC have been excavated in Mexico and the Indus Valley Civilization. During the late medieval period, cotton became known as an imported fiber in Europe.

90% of Angora fur is produced in China, where more than 50 million Angora rabbits grow 2,500–3,000 tonnes of angora wool per year. Angora is known for its softness, silky texture, and what knitters refer to as a halo (fluffiness). It is much warmer and lighter than wool due to the hollow core of the angora fibre.

Glass-ceramic materials share many properties with both non-crystalline glass and crystalline ceramics. The negative thermal expansion of the crystalline ceramic phase can be balanced with the positive expansion of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net expansion near zero and can sustain repeated and quick temperature changes up to 1000 °C.

The traditional metal for church bells is a bronze of about 23% tin. Steel was tried during the busy church-building period of mid-19th-century England, for its economy over bronze, but was found not to be durable.

Diamond is the hardest substance in the world. Hence it can be cut only by using a diamond.