How your eyes see color, why cats purr, and the tower that refuses to fall

A self-driving car has to do what a human driver does: see the road, predict what everyone else will do, and choose how to steer, speed up or brake, many times every second.

To see, these cars carry a ring of sensors. Cameras read lane lines, signs and traffic lights. Radar measures how fast other cars are moving, even in rain or fog. Many also use lidar, which sweeps the scene with pulses of laser light to build a 3D map.

A powerful onboard computer blends all of that into one picture of the world, then plans a safe path. Engineers train these systems on millions of miles of driving, including rare and tricky situations.

Lidar measures distance by timing how long laser light takes to bounce back.

Engineers describe automation in six levels, from 0 (no automation) to 5 (a car that can drive anywhere a person could). Many new cars already help with steering and braking, and driverless taxis now carry passengers in a few U.S. cities.

A 3D printer for houses works a lot like a giant frosting bag. A computer-guided nozzle, riding on a frame or a robotic arm, squeezes out a thick ribbon of a concrete-like mix and traces the outline of every wall.

When one layer is done, the nozzle rises and lays the next one on top. Layer by layer, the walls grow from the ground up, with gaps left for doors, windows, pipes and wiring.

Because the machine follows a digital plan, it puts material only where it is needed, so there is less waste. Curved walls, which are slow and costly to build by hand, are just as easy for a printer as straight ones.

The printer can lay down a home's walls in days, not weeks.

People still do plenty of the work. Crews add the roof, windows, plumbing and electrical systems. Builders hope printing can help make homes faster and more affordable, and printed houses and neighborhoods already stand in several countries.

Hurricanes start over warm tropical oceans, where the water is at least about 80°F (27°C). Warm, moist air rises from the sea surface, cools and forms towering thunderclouds. As the air rises, it leaves lower pressure behind, and more air rushes in to replace it.

Because Earth spins, that inflowing air curves as it travels. North of the equator the storm turns counterclockwise; south of it, clockwise.

Once the winds reach 74 miles per hour, a tropical storm officially becomes a hurricane. At its heart is the eye, an area of light winds and often clear sky, wrapped by the eyewall, where the strongest winds blow.

The calmest spot in a hurricane is right in the middle.

Forecasters rate hurricanes from Category 1 to 5; a Category 5 has winds of 157 mph or more. The same kind of storm is called a typhoon in the northwestern Pacific and a cyclone in the Indian Ocean. The Atlantic hurricane season runs from June 1 to November 30.

Light enters your eye through the pupil, and the lens focuses it onto the retina, a thin layer at the back of the eye packed with light-sensing cells.

There are two main kinds. Rods, about 120 million of them, work in dim light but see only shades of gray. That is why colors fade at night. Cones, about 6 million, need brighter light and give us color.

Most people have three types of cones. One responds best to long wavelengths (reds), one to medium (greens) and one to short (blues). Your brain compares their signals and mixes them into every shade you see.

Yellow is your brain's reading of red and green cones firing together.

If one type of cone is missing or works differently, a person is color blind, most often between reds and greens. It is passed down through families and affects about 1 in 12 men and 1 in 200 women. Regular eye exams catch many vision problems early.

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

Jupiter's Great Red Spot is a storm wider than Earth. It has been swirling for at least 150 years.

The Moon has quakes. Instruments left by Apollo astronauts recorded thousands of moonquakes.

A polar bear's skin is black. Its fur is see-through and only looks white because it scatters light.

A group of crows is called a murder. Owls come in a parliament, and flamingos in a flamboyance.

Earth's inner core is roughly as hot as the surface of the Sun, about 5,000°C (9,000°F) or more.

Honeybees give directions by dancing. A waggle dance tells hive-mates which way to fly to find flowers and how far.

The famous tower in Pisa, Italy, is the bell tower of the city's cathedral. Work began in 1173, and it was never meant to lean.

The trouble was underground. The tower stands on soft ground of clay, sand and silt, and one side settled more than the other. By the time builders reached the third floor, the tower had already started to tip.

Then wars stopped the work for almost a century. That long pause probably saved it, because it gave the soil time to settle and firm up. Later builders tried to correct the lean by making upper floors slightly taller on one side, which left the tower a little curved. The bell chamber on top was finished in the 1370s.

Engineers saved the tower by carefully removing soil from under its high side.

By 1990 the lean had grown to about 5.5 degrees, and the tower was closed to visitors. Over the next decade, engineers gently pulled out soil from under the north side so the tower could settle back. It reopened in 2001, leaning about 4 degrees, and is expected to stay safe for centuries.

In the 1860s, crossing the United States by wagon could take months. The answer was a railroad linking the East's rail network to the Pacific coast.

Two companies raced toward each other. The Central Pacific built east from Sacramento, California, blasting through the granite of the Sierra Nevada. The Union Pacific built west across the Great Plains from the Missouri River.

Thousands of Chinese immigrants did much of the hardest and most dangerous work on the Central Pacific. Irish immigrants and Civil War veterans made up much of the Union Pacific's crews.

When the last spike went in, a telegraph flashed one word across the country: DONE.

The two lines met at Promontory Summit, in Utah Territory, on May 10, 1869, where a ceremonial golden spike was tapped into place. A coast-to-coast trip that once took months now took about a week.

Mount Everest is the highest mountain on Earth above sea level. A 2020 survey by Nepal and China put its summit at 8,849 meters, or about 29,032 feet, higher than airliners sometimes cruise.

It sits in the Himalayas on the border between Nepal and the Tibet region of China. In Nepal it is called Sagarmatha, and in Tibet, Chomolungma.

The Himalayas formed when the plate carrying India crashed into Asia, and they are still being pushed up a few millimeters a year. The rock near the summit holds fossils of sea creatures, because it was once the floor of an ancient ocean.

The top of the world was once the bottom of the sea.

On May 29, 1953, Edmund Hillary of New Zealand and Tenzing Norgay, a Sherpa from Nepal, became the first people known to reach the summit. The air up there holds only about a third as much oxygen as at sea level, so most climbers carry bottled oxygen.

The Galápagos Islands sit on the equator in the Pacific Ocean, about 600 miles (1,000 km) off the coast of Ecuador, the country they belong to. They are the tops of volcanoes, and some are still active.

Because the islands were never joined to the mainland, the animals that reached them evolved on their own. Many live nowhere else, and most show little fear of people.

Giant tortoises can weigh more than 500 pounds and live well over 100 years; the islands are named after them. Marine iguanas are the only lizards that dive into the sea to graze on algae. Blue-footed boobies show off their bright feet in a high-stepping dance.

Darwin's 1835 visit helped spark his ideas about how species change.

Most of the islands are a national park, and in 1978 the Galápagos were among the first places named a UNESCO World Heritage Site. Visitors stay on marked trails, always with a licensed guide.

Stand on a beach for a few hours and the waterline creeps up or slides away. Those are the tides, and the Moon is mostly responsible.

The Moon's gravity pulls on Earth's oceans, piling water into a bulge on the side facing the Moon. A second bulge forms on the far side of Earth, where the pull is weakest. As Earth turns through both bulges, most coasts get two high tides and two low tides a day.

Because the Moon moves along its orbit, the tides arrive about 50 minutes later each day. The Sun pulls on the oceans too. When Sun, Moon and Earth line up at new and full moon, the tides are extra strong: spring tides, a name that has nothing to do with the season.

The world's biggest tides can rise more than 50 feet.

The shape of a coast matters. In Canada's Bay of Fundy, water funnels into a long, narrow bay and the difference between high and low tide can top 50 feet (16 m), the highest anywhere.

A purr comes from the voice box, or larynx. Tiny muscles there twitch very fast, opening and closing the gap between the vocal cords. Air rushing through on every breath, in and out, makes that steady rumble.

The buzz runs at roughly 25 to 150 vibrations a second. Kittens start purring when they are only a few days old, which may help them signal to their mother while nursing.

Most of us hear a purr as a happy sound, and often it is. But cats also purr when they are frightened, hurt or giving birth, so scientists think a purr can also be a way to calm down or ask for care.

A purr is not always a sign that a cat is content.

Researchers are studying whether purring's low vibrations might help a cat's bones and muscles heal, but that idea is not proven. House cats, bobcats, cougars and cheetahs purr, while most big cats that roar, like lions and tigers, cannot purr the way a house cat does.

Before chemistry labs, every color in a painting had to come from something you could dig, crush or boil. Some were cheap, like earth reds and browns. Others were treasures.

The deep blue called ultramarine, meaning "beyond the sea," was ground from lapis lazuli, a stone mined in the mountains of Afghanistan. In Renaissance Europe it could cost more than gold, so painters saved it for the most important parts of a picture.

Tyrian purple, worn by Roman emperors, came from sea snails. It took thousands of them to dye a single garment, which is why purple became the color of royalty.

Two of art's great colors were discovered by mistake.

Around 1706 in Berlin, a paint maker trying to make red ended up with Prussian blue, one of the first modern man-made pigments. And in 1856, an 18-year-old chemist named William Henry Perkin was trying to make a malaria medicine when he created mauve, the first synthetic dye. A rainbow of cheap, bright colors followed.

Early movies were black and white, but filmmakers wanted color from the start. Some early films were hand-painted frame by frame, a slow job done with tiny brushes. Others were dipped in dye to give whole scenes a blue night or an orange glow.

Real color photography on film was harder. The breakthrough was the three-strip Technicolor camera of the 1930s.

Inside it, a prism split the light coming through the lens into three beams, each recorded on its own strip of black-and-white film: one for red, one for green, one for blue. Those three records were then combined with dyes to make a single full-color print. It is the same three-color idea your eyes use.

The camera saw color the way your eye does: in red, green and blue.

Becky Sharp (1935) was the first feature-length film made entirely in three-strip Technicolor. In The Wizard of Oz (1939), the story starts in sepia tones, then bursts into color when the heroine opens the door to Oz.