Why we dream, how Stonehenge's giant stones got there, and the falcon that dives at 200 mph

About 30 GPS satellites circle Earth roughly 20,000 km up, each going around twice a day. Every one of them carries extremely precise atomic clocks, and every one keeps broadcasting a simple message: who it is, where it is, and exactly what time it is.

Your phone does not send anything back. It just listens. Because the signals travel at the speed of light, the tiny delay in each one tells your phone how far away that satellite is.

Know your distance from one satellite and you could be anywhere on a giant sphere. Add a second and a third, and the spheres cross at a single point. A fourth satellite fixes the small error in your phone's own clock, which is far less accurate than the ones in space.

A timing error of just one millionth of a second would put you about 300 meters off.

The engineers even correct for Einstein's relativity: clocks in orbit tick at a slightly different rate than clocks on the ground.

Every night your brain runs through several sleep cycles, each lasting about 90 minutes. The most vivid, story-like dreams come during REM sleep, named for the rapid eye movements that happen under closed eyelids.

During REM your brain is almost as active as when you are awake, while most of your muscles are temporarily switched off. Scientists think that is what stops you from acting out your dreams.

Nobody knows for sure why we dream, but there are strong leads. Dreams seem to help sort and store memories, process emotions, and perhaps rehearse how to handle threats in a safe setting.

Most people dream several times a night and forget nearly all of it within minutes of waking.

Your gut is home to tens of trillions of microbes, mostly bacteria, made up of hundreds of different species. Together they are called the gut microbiome, and scientists now think of it almost like an extra organ.

These tiny tenants help break down food your own body cannot digest, especially fiber. In return they make useful substances, including some vitamins, such as vitamin K.

Researchers are still working out how much the microbiome affects the rest of the body, from the immune system to mood. What is clear is that a varied diet tends to support a more varied community of microbes.

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

Scotland's national animal is the unicorn. It has appeared on Scottish royal coats of arms for centuries.

Koalas have fingerprints so similar to ours that they can be hard to tell apart under a microscope.

The shortest war on record was between Britain and Zanzibar in 1896. It was over in under an hour.

A teaspoon of neutron star would weigh billions of tons on Earth. These collapsed stars pack more than the Sun's mass into a ball the size of a city.

The Moon is drifting away from Earth by about 3.8 cm a year, roughly the speed your fingernails grow.

Butterflies taste with their feet. Sensors on their legs tell them if a leaf is right for their eggs.

Stonehenge stands on Salisbury Plain in southern England. It was built in stages, starting about 5,000 years ago with a circular ditch and bank. The giant stones went up a few centuries later, around 2500 BC, built by Neolithic farmers with no metal tools and no wheels.

The huge sarsen stones weigh about 25 tons on average. A 2020 study traced most of them to West Woods, about 25 km (15 miles) to the north.

The smaller bluestones traveled much farther: about 240 km (150 miles) from the Preseli Hills in Wales. In 2024, scientists traced the flat Altar Stone at the center to northern Scotland, more than 700 km away.

How did they move them? Most likely with wooden sledges, ropes, and a lot of people.

The monument lines up with the sunrise on the summer solstice and the sunset on the winter solstice, and thousands still gather there to watch.

The Mariana Trench is a long, curved scar in the floor of the western Pacific Ocean, near the island of Guam. It formed where one giant plate of Earth's crust slides beneath another.

Its deepest point, called the Challenger Deep, is nearly 11,000 meters (about 36,000 feet) below the surface. Drop Mount Everest in, and its peak would still be more than 2 km underwater.

Down there it is pitch black and near freezing, and the water pressure is more than 1,000 times what you feel at sea level. Yet life thrives: scientists have found tiny shrimp-like animals, and snailfish have been filmed more than 8,000 meters down.

The Sun constantly blows out a stream of charged particles called the solar wind. Earth's magnetic field steers some of them toward the poles, where they slam into the upper atmosphere and make the gases there glow.

That glow is the aurora: the northern lights (aurora borealis) in the north and the southern lights (aurora australis) in the south.

The color depends on which gas is hit, and how high up. Oxygen gives the common green glow, and a rarer deep red higher up. Nitrogen adds blues and purples along the lower edges.

The light shows happen about 100 km or more above your head, far higher than planes fly.

Big solar storms can push auroras far from the poles. In May 2024, they were seen across much of the United States.

On land: the cheetah can hit about 100 to 110 km/h (around 65 mph), but only in short sprints of a few hundred meters. Its flexible spine works like a spring, and its long tail helps it steer.

The long-distance runner: North America's pronghorn is a little slower, but it can keep up a fast pace for miles. A cheetah would give up long before.

In the air: the peregrine falcon is the fastest animal on Earth. In a hunting dive, called a stoop, it can top 300 km/h, or about 200 mph.

At sea: speeds are hard to measure underwater, so estimates vary a lot, but the sailfish is usually named the fastest fish. It folds its huge fin flat to cut through the water.

The fastest human sprinters top out at about 44 km/h.

Around 1831, the Japanese artist Katsushika Hokusai, then in his seventies, published a print of a giant wave curling over three small boats, with Mount Fuji tiny in the distance. It is now one of the most recognized images in the world.

It was not a one-off painting. It was a woodblock print: the design was carved into blocks of wood, one for each color, then inked and pressed onto paper again and again.

That means thousands of copies were sold, cheaply, to ordinary people. Original impressions survive in museums across the world today.

The print was part of a series called Thirty-six Views of Mount Fuji, and it helped spark a craze for Japanese art in Europe.