How volcanoes blow, why leaves turn red, and the sea creatures that make their own light

Sound is a wave of pressure moving through the air: tiny pushes and pulls, peaks and troughs. Noise-cancelling headphones fight those waves with more waves.

Small microphones on the ear cups listen to the world around you. A chip inside then creates a new sound wave that is the exact mirror image of the noise, with a trough wherever the noise has a peak.

When the two waves meet, they cancel each other out. Scientists call this destructive interference. What reaches your ear is much quieter than what came in.

The trick works best on steady, low hums, like a plane engine or an air conditioner.

Sudden or high-pitched sounds, such as voices or a clattering cup, are harder to predict, so the soft padding has to block those. The basic idea was patented back in the 1930s, and pilots were among the first big users.

Concrete is the most widely used building material on Earth. It is also prone to cracking. Water seeps into the cracks, rusts the steel bars inside, and slowly weakens bridges, tunnels and buildings. Repairs cost a fortune.

One fix being tested sounds like science fiction: concrete with bacteria built in. Engineers mix in dormant bacteria that can survive for years, along with a food supply for them.

When a crack opens and rainwater gets in, the bacteria wake up, eat, and produce limestone. The limestone fills the crack and seals it, with no repair crew needed. It works on fine, hairline cracks rather than big breaks.

The ancient Romans may have got there first.

A 2023 study found that small lumps of lime in Roman concrete can react with water and fill cracks, which may help explain why Rome's Pantheon dome still stands after nearly 2,000 years.

Deep underground, rock can get hot enough to melt into magma. Because magma is lighter than the solid rock around it, it slowly rises and collects in pools called magma chambers, a few kilometers below the surface.

Magma also holds dissolved gases, mostly water vapor and carbon dioxide. Think of a bottle of soda: when the pressure drops, the gas fizzes out. When magma nears the surface, its gas does the same.

What happens next depends on the magma. Runny magma lets gas escape easily, so it oozes out as rivers of lava, as in Hawaii. Thick, sticky magma traps the gas until it blasts free in an explosion, as at Mount St. Helens in 1980.

Once magma reaches the surface, we call it lava.

Most volcanoes sit where Earth's giant plates pull apart or push together. The rim of the Pacific is so busy it is called the Ring of Fire.

Bones can look like dry sticks, but they are living tissue with their own blood supply and nerves. They also make blood: red and white blood cells are produced in the marrow inside many bones.

Your skeleton is constantly being rebuilt. Some cells break down old bone while others lay down new bone. Over roughly ten years, an adult skeleton is largely replaced.

Babies are born with around 300 bones or bone pieces. Many fuse as we grow, leaving adults with 206. More than half of them are in the hands and feet.

Most people reach their peak bone strength by about age 30.

General information, not medical advice. Talk to a doctor about your own health.

Saturn would float in a bathtub big enough to hold it. On average the planet is less dense than water.

Peanuts are not nuts. They are legumes, in the same family as beans and peas, and they grow underground.

Your heart beats about 100,000 times a day, pumping blood through every part of your body.

The dot over a lowercase i or j has a name: it is called a tittle.

Alaska is the westernmost and the easternmost U.S. state. Its Aleutian Islands stretch across the 180° line of longitude.

Hummingbirds can fly backwards. Their wings beat so fast they hum, which is how they got their name.

The Great Wall of China is really a network of walls, built and rebuilt over about 2,000 years. The earliest sections went up in the 7th century BC, when rival Chinese states built walls to guard against one another.

After China's first emperor, Qin Shi Huang, unified the country in 221 BC, he had many of those walls joined to defend against raiders from the north.

Most of the brick-and-stone wall that visitors walk on today was built much later, by the Ming dynasty between the 1300s and 1600s. Earlier sections were often made of packed earth.

A 2012 Chinese survey measured all the branches at about 21,000 km (13,000 miles).

One popular myth is false: the wall is not visible to the naked eye from the Moon, and it is very hard to spot even from low orbit. It became a UNESCO World Heritage Site in 1987.

In 1799, French soldiers rebuilding a fort near the town of Rashid, which Europeans called Rosetta, in Egypt's Nile Delta, found a broken slab of dark stone covered in writing.

The stone carries a decree issued by priests in 196 BC, honoring the young king Ptolemy V. The same message is written three times: in hieroglyphs, in an everyday Egyptian script called Demotic, and in ancient Greek.

By then nobody had been able to read hieroglyphs for well over a thousand years. But scholars could read Greek, so the stone was a key. The British took it in 1801, and it has been in the British Museum in London since 1802.

In 1822, Jean-François Champollion announced he had cracked hieroglyphs.

He built on earlier work by the English scientist Thomas Young, and showed that hieroglyphs mix sound signs with picture signs. Thousands of years of Egyptian writing could finally be read.

The Amazon carries more water than any other river on Earth, about one-fifth of all the river water that flows into the world's oceans. It pours out more than the next several biggest rivers combined.

It runs roughly 6,400 km (4,000 miles) from the Andes mountains of Peru to the Atlantic coast of Brazil. Whether it or the Nile is the world's longest river is still argued over, because it depends where you measure from.

Its basin covers about 40% of South America and holds the largest tropical rainforest on the planet, home to an enormous share of the world's plant and animal species.

Not a single bridge crosses the main river.

Most of the land it flows through is thinly populated forest and wetland, and the banks shift with the seasons, so boats do the work that bridges do elsewhere. In the wet season, parts of the river swell many kilometers wide.

Hidden in the desert mountains of southern Jordan, Petra was the capital of the Nabataeans, a wealthy trading people. More than 2,000 years ago they carved temples, tombs and homes straight into rose-colored sandstone cliffs.

Most visitors arrive through the Siq, a narrow, winding gorge about 1.2 km long with rock walls towering on both sides. At its end, the Treasury suddenly appears: a carved facade about 40 meters tall.

The Nabataeans were expert water engineers, using channels, dams and cisterns to keep a city alive in the desert. After a major earthquake in AD 363 and shifting trade routes, Petra slowly emptied. A Swiss explorer brought it to wide European attention in 1812.

All summer, leaves are packed with chlorophyll, the green pigment plants use to turn sunlight into food. There is so much of it that it hides the other colors in the leaf.

As days shorten and nights cool, trees stop making chlorophyll and it breaks down. The yellows and oranges come from pigments called carotenoids, the same family that colors carrots. They were in the leaf all along.

Reds and purples are different. Trees such as many maples make new pigments, called anthocyanins, in the fall. Sunny days followed by cool but not freezing nights tend to give the brightest reds.

Brown leaves are what remains once the other pigments are gone.

Meanwhile, a layer of cells grows where each leaf meets its twig and slowly seals it off. Dropping its leaves helps the tree save water through the winter.

Some living things can glow, a talent called bioluminescence. Inside their bodies, a chemical called luciferin reacts with oxygen, helped along by an enzyme called luciferase. The result is light with almost no heat, often called cold light.

On land, fireflies are the famous example: they flash patterns to find mates. In some caves in New Zealand, glow-worms, which are really insect larvae, hang sticky threads lit up like stars to trap flying insects.

The real kingdom of light is the ocean. A large survey off California found that roughly three out of four deep-sea animals can glow. Most of that light is blue or green, the colors that travel farthest through seawater.

The anglerfish dangles a glowing lure in the dark, and dinner swims right up to it.

Other animals use light to hide, startle attackers or call for help. Tiny plankton can make breaking waves sparkle blue at night.

Michelangelo thought of himself as a sculptor, not a painter. Yet in 1508, Pope Julius II hired him to paint the ceiling of the Sistine Chapel in the Vatican. He worked on it for about four years, finishing in 1512.

Despite the famous image, he did not paint lying on his back. He stood on a wooden scaffold he designed himself, with his head tipped back for hours. He even wrote a joking poem about how much it hurt his neck.

The ceiling is a fresco: paint brushed onto wet plaster, so the color becomes part of the wall as it dries. That meant painting fast, one fresh patch at a time, before the plaster set.

More than 300 figures cover the ceiling, telling stories from the Book of Genesis.

The most famous scene, the Creation of Adam, shows two hands almost touching. A long cleaning finished in the 1990s revealed colors far brighter than centuries of candle smoke had let people see.

Many scenes you see in movies, and the weather map behind your local forecaster, are filmed in front of a plain, brightly lit wall of a single color. Software then picks out every pixel of that color and swaps in a different background. The technique is called chroma key.

Green is the usual choice because it is about as far from human skin tones as a color can be, so faces and hands are not accidentally erased.

Digital cameras help too. Most camera sensors have twice as many green-sensing pixels as red or blue ones, so green comes through bright and clean, with little noise.

Blue screens still get used, especially when a costume or prop is green.

The golden rule for actors: never wear the key color. A green tie in front of a green screen means a hole right through your chest.