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Cactuses have spines, which are special, modified leaves. Thorns come from stem tissue, while prickles are sharp extensions of the plant’s skin. And horns are growths on animals that are often made of bone and covered in a material called keratin.

The first suns in the Universe were Population III stars. Though nobody has seen one yet, they are thought to be stars that formed before metals were produced in the Universe. Stars we can see nearby contain metal produced by the death of these stars. These ‘high metal’ stars, like our Sun, are called Population I.

Retinopathy affects a person’s retina, which is a layer of tissue at the back of each eyeball. The retina senses light and communicates these signals to the brain.

True. In March 2025, the International Astronomical Union recognised 274 moons in Saturn’s orbit, compared with Jupiter’s 95 known moons. Most of these are small lumps of rock, probably created by the break-up of larger rocks captured by Saturn’s gravity.

A23a is the world’s largest iceberg. Twice the size of London, this trillion-ton chunk of ice broke free from Antarctica’s Filchner-Ronne Ice Shelf in 1986, before running aground for over a quarter of a century. In 2020 it started to move again, only barely avoiding collision with a penguin sanctuary in 2025.