1) The ozone layer, located in the Earth's stratosphere, protects us from harmful ultraviolet (UV) radiation.

2) The world's largest flower is the Rafflesia arnoldii, which can grow up to 3 feet (1 meter) in diameter and weighs about 15 pounds (7 kilograms).

3) The invention of the printing press by Johannes Gutenberg in the 15th century revolutionized the spread of knowledge and played a key role in the Renaissance.

4) The Mona Lisa, painted by Leonardo da Vinci, is one of the most famous and valuable artworks in the world.