How planes stay up, why onions make you cry, and the secret winter of a sleeping bear

The James Webb Space Telescope launched on Christmas Day, 2021. It is the largest telescope ever sent into space, and it was built to see what older telescopes could not.

Its main mirror is 6.5 meters (about 21 feet) across, made of 18 six-sided segments coated with a thin layer of gold. Gold reflects infrared light very well, and infrared is what Webb is designed to see.

Why infrared? Light from the most distant galaxies has traveled for billions of years, and the expanding universe has stretched it into infrared. Looking that far away means looking back in time, toward the first galaxies.

The mirror was too big for any rocket, so it launched folded up.

Webb works far from Earth, about 1.5 million kilometers away. A five-layer sunshield roughly the size of a tennis court keeps its instruments extremely cold, so the telescope's own heat does not blur its view.

About 97% of the water on Earth is salty ocean water, and much of the fresh water is locked up in ice. As cities grow and droughts bite, more places are turning to the sea itself.

The process is called desalination. The oldest method is simple: boil salt water and collect the steam, which leaves the salt behind. It works, but heating huge amounts of water takes a lot of energy.

Most new plants use reverse osmosis instead. Pumps push seawater at very high pressure through special membranes with holes so tiny that water molecules squeeze through but salt does not.

The challenge is cost: pushing water through those filters takes a lot of power.

Engineers are working on tougher membranes, solar-powered plants and better ways to handle the leftover extra-salty water, called brine, so it does not harm sea life.

A loaded passenger jet can weigh hundreds of tons, yet it floats along at around 35,000 feet. The secret is in the wings, and in four forces that are always at work: lift, weight, thrust and drag.

Engines provide thrust, pushing the plane forward. Air resists, which is drag. Gravity pulls the plane down, which is weight. To fly, the wings must make lift, an upward push, at least as big as the weight.

A wing is curved and tilted slightly upward as it slices through the air. That shape and angle bend the flowing air downward. Push air down, and the air pushes the wing up, just as Isaac Newton's laws predict.

Air moving over the top of the wing speeds up and its pressure drops.

Higher pressure underneath and lower pressure on top add up to lift. The faster a plane moves, the more lift its wings make, which is why jets race down a long runway before they rise.

An onion sitting on the counter will not make you cry. The trouble starts when you cut it. Slicing breaks open the onion's cells, and chemicals that were kept apart suddenly mix.

Enzymes turn the onion's sulfur compounds into a new substance that drifts into the air as a gas. When that gas reaches your eyes, it irritates the nerves on their surface.

Your eyes respond the only way they can: tear glands flood them to wash the irritant away. These are called reflex tears, the same kind you make when dust or smoke gets in your eyes.

It is the onion's defense system, meant to put off animals that bite it.

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

Sunlight is about eight minutes old when it reaches you. That is how long light takes to cross the roughly 150 million kilometers from the Sun.

The largest desert on Earth is Antarctica. A desert is a place with very little rain or snow, not necessarily a hot one.

Strawberries are not true berries, at least to a botanist. Grapes and tomatoes are.

A fluffy cloud can weigh about a million pounds. Its water is spread out as tiny droplets, so it floats.

The word alphabet comes from alpha and beta, the first two letters of the Greek alphabet.

Cats cannot taste sweetness. They lack the taste receptor that lets us enjoy sugar.

Work on the Colosseum began around AD 70 under Emperor Vespasian. His son Titus opened it in AD 80 with 100 days of games. Its official name was the Flavian Amphitheater, after their family.

It was enormous for its time: an oval about 189 meters long and nearly 50 meters tall, built from stone, brick and Roman concrete. Around 50,000 spectators could pack inside.

Crowds came to watch gladiator fights, wild animal hunts and grand public shows. Under the wooden arena floor was a maze of tunnels and rooms, where lifts and trapdoors could send animals and scenery up into the light.

With 80 entrances, the whole crowd could get in and out quickly.

Earthquakes and centuries of people taking its stone for other buildings left it partly ruined. Today it is one of the most visited places in Italy and a symbol of Rome.

On July 16, 1969, three astronauts launched from Florida: Neil Armstrong, Buzz Aldrin and Michael Collins. Four days later, Armstrong and Aldrin guided their lunar module, the Eagle, down onto a flat plain called the Sea of Tranquility.

Collins stayed in orbit in the command module, Columbia, waiting to bring them home.

Armstrong climbed down the ladder first, and Aldrin followed about 20 minutes later. Hundreds of millions of people watched the grainy black-and-white pictures live on TV.

The pair spent about two and a half hours walking on the Moon.

They collected about 22 kilograms of moon rock, set up experiments and planted a flag. All three splashed down safely in the Pacific Ocean on July 24. With no wind on the Moon, their footprints are probably still there.

The Sahara is the largest hot desert in the world. It covers about 9 million square kilometers of North Africa, roughly the size of the United States, and stretches across about ten countries.

Picture it and you probably see giant sand dunes. Some do rise more than 150 meters high, but much of the Sahara is actually bare rock, gravel plains and mountains.

Days can be scorching, yet nights can turn cold, because dry air and clear skies let the heat escape fast.

Thousands of years ago, the Sahara was green.

Ancient rock paintings there show giraffes, cattle and swimmers, from a time when the region had lakes and grassland. Today, wind carries Saharan dust all the way across the Atlantic, where it helps feed plants in the Amazon rainforest.

High in Peru's Andes mountains, about 2,430 meters (nearly 8,000 feet) above sea level, sits Machu Picchu. The Inca built it in the mid-1400s, most likely as an estate for the emperor Pachacuti.

Its builders cut granite blocks so precisely that many fit together without any mortar. The tight joints help the walls ride out earthquakes.

The site was abandoned roughly a century later. The Spanish conquerors never found it. Local farmers knew it was there, but the wider world took notice after the explorer Hiram Bingham publicized it in 1911.

Terraces carved into the slopes once grew crops and still stop landslides.

Going? Tickets are timed and limited, so book early. Many visitors arrive by train and bus from Cusco. Hikers can take the multi-day Inca Trail, which needs a permit and a licensed guide.

A snowflake starts high in a cold cloud, when water vapor freezes onto a speck of dust or pollen. That tiny seed becomes an ice crystal.

Water molecules lock together in a six-sided pattern when they freeze. That is why almost every snowflake has six sides or six arms, as the crystal grows by grabbing more water vapor from the air.

The exact shape depends on the temperature and the moisture the flake meets as it falls. Some conditions grow flat plates, others thin needles or columns, and others the classic feathery stars.

Ice is clear, but snow looks white because its many crystal faces scatter light in every direction.

In 1885, a Vermont farmer named Wilson Bentley became the first person known to photograph a single snowflake. He went on to capture thousands, and he never found two exactly alike.

In late summer and fall, bears go on an eating spree, piling on fat. Some take in as many as 20,000 calories a day. That fat is the fuel for a long winter in a den.

Once inside, a black bear in a cold northern forest may stay for five months or more without eating, drinking, peeing or pooping. Its heartbeat slows dramatically and its breathing becomes slow and shallow.

Bears have a remarkable trick: their bodies recycle waste that would normally leave as urine, turning it back into protein. That helps them keep much of their muscle through the long fast.

Mother bears give birth in the middle of winter, in the den.

Black bear cubs weigh well under a pound at birth, about the size of a squirrel. They drink their mother's milk while she rests, and by spring they are ready to follow her out. Because a bear's body stays fairly warm, it can wake up quickly if disturbed.

Origami is the art of folding paper into shapes. The word is Japanese: ori means folding, and kami means paper. In classic origami, a single square sheet becomes a bird, a flower or a frog, often with no cutting and no glue.

The best-known model is the crane. A Japanese tradition says that anyone who folds 1,000 paper cranes may be granted a wish, and strings of a thousand cranes are given as gifts of hope and healing.

Modern folders have pushed the art much further, designing insects with legs and antennae, and animals with fur-like texture, all from uncut paper. Many work out their designs with geometry first.

Engineers use origami to pack big things into small spaces.

Folding tricks inspired by origami help squeeze solar panels and other large parts of spacecraft into rockets, ready to unfold once they reach space.

In stop-motion animation, nothing on screen really moves. Animators pose a puppet or clay figure, take a single photo, nudge it a tiny bit, and take another. Played back quickly, the still photos blend into smooth motion.

Movies usually run at 24 frames per second. Animators often shoot each pose twice, so one second of film can still take 12 separate poses.

The puppets usually hide a metal skeleton, called an armature, so they hold each pose without flopping. Faces may be swapped piece by piece to change expressions.

A good day's work might be just a few seconds of finished film.

The technique is as old as cinema itself. The 1933 film King Kong brought its giant ape to life with a small stop-motion model, and the craft is still loved today for its handmade, real-world look.