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At the summit of Mount Everest, 29,032 feet up, the air holds the same 20.9 percent oxygen as at sea level, yet breathing it can kill. This episode explains the death zone, which begins at 8,000 meters, about 26,246 feet, and covers only 14 peaks in the Himalaya and Karakoram. Swiss doctor Edouard Wyss-Dunant first called it the lethal zone in 1953. The problem is pressure: it falls from about 1,013 millibars at sea level to under 356, too little to push oxygen across the lungs into hemoglobin. The body fights back with EPO and extra red blood cells, thickening the blood and shutting down digestion.

Above 8,000 meters acclimatization stops working and the body burns oxygen faster than it can be replaced. Fluid leaks into the lungs and brain in HAPE and HACE, and failing judgment shaped disasters like Everest in 1996 and K2 in 2008. Researchers in Bolivia point out that a fetus develops at oxygen pressure effectively identical to the Everest summit, thanks to sticky fetal hemoglobin. The episode then traces the oxygen debates, Griffith Pugh's 1953 protocol, and the climb that physiologists said was impossible.

• The highest permanent human habitation cited is 5,950 meters, where someone lived for two years
• Pugh's four week acclimatization plan, timed to red blood cell production
• Tom Bourdillon and Charles Evans turned back about 300 feet short when moisture froze their closed circuit oxygen valves
• Edmund Hillary grew clumsy and slow within 10 minutes of removing his mask on the summit
• Reinhold Messner and Peter Habeler reached the top without supplemental oxygen in 1978

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In May 2018 a one second audio clip divided the internet: in a Twitter poll of more than 500,000 people, 53 percent heard Laurel and 47 percent heard Yanny, and each side found the other impossible to understand. The recording began in 2007, when opera singer Jay Aubrey Jones recorded about 200,000 reference pronunciations for Vocabulary.com in a makeshift New York apartment studio. Eleven years later Katie Hetzel, a 15 year old freshman at Flowery Branch High School in Georgia, looked up laurel, heard Yanny, and shared the clip by replaying it through speakers and re-recording it in a noisy classroom.

From Instagram to Reddit to YouTuber Chloe Feldman on Twitter, the clip exploded, drawing in Ellen DeGeneres, Stephen King, Chrissy Teigen and the White House. Spectrograms showed both words living in the same degraded file: Laurel in the low frequencies and Yanny in high frequency noise. Age-related loss of high frequency hearing explains part of the split, but the episode also follows how the brain forces a single answer from an ambiguous signal and how researchers flipped listeners' perception without touching the clip.

• Researchers recreated the Yanny pattern by splicing syllables from the same singer's recordings of other words, including uncanny
• Audiology professor Benjamin Munson mapped Yanny to the high frequencies and Laurel to the low
• The New York Times pitch slider that turned the clip into clear Laurel or clear Yanny
• Hans Rutger Bosker's study where a muffled lead-in sentence primed listeners to hear Yanny
• Look-alike illusions: the Brainstorm or green needle toy and Grover's misheard line on Sesame Street

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Bored office workers yawn, and so do paratroopers seconds before they jump. This episode explores chasmology, the study of yawning, starting with the reflex itself: a slow inhalation, a peak of involuntary muscle stretching called the acme, then a fast exhalation, a sequence seen in fetuses on 4D ultrasound at 30 weeks. The airway dilates to three or four times its size, and the rumble in your head is the tensor tympani muscle in the middle ear. The old idea that yawning pulls in oxygen fails the test: changing oxygen or carbon dioxide levels leaves yawn frequency unchanged.

The leading explanation is brain cooling. At SUNY Albany, Gordon Gallup's team found that people with cold packs on their foreheads, or breathing only through their noses, caught far fewer contagious yawns. Yawning also raises cortisol, which may explain the skydiver. Contagion follows emotional closeness, from kin to strangers, appears around age six, and even crosses to dogs. Elsewhere in the animal kingdom yawns are threats, courtship and jaw resets, while human cultures treated them as an open door for spirits and plague.

• A University of Pisa study ranking who catches yawns fastest: family, then friends, then acquaintances, then strangers
• Puppies only start catching their owners' yawns at about seven months old
• Baboons, guinea pigs and Siamese fighting fish yawn to display weapons, while Adélie and emperor penguins yawn while courting
• Polydore Vergil's account of people making the sign of the cross over yawning mouths during the plague
• George Washington's civility rule to yawn privately behind a hand, and judges who can hold loud yawners in contempt

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That maddening moment when a word you know refuses to come out has a clinical name, lethologica, and a long research history. William James described its mild anguish in his 1890 book The Principles of Psychology, and Sigmund Freud blamed repressed thoughts. The empirical breakthrough came in 1966, when Harvard researchers Roger Brown and David McNeill read people definitions of obscure words like sampan, apocrypha and nepotism. Participants stuck on the tip of the tongue could guess the first letter, the syllable count and the stress pattern far better than chance, showing the brain really does hold the word even when the mouth cannot say it.

The episode weighs the transmission deficit model, where meaning and sound live in separate networks and the signal between them fades, against the inferential view, where the brain only infers that it knows the word. Brain scans show the anterior cingulate cortex and right dorsolateral prefrontal cortex lighting up as a conflict alarm, and a lorazepam study found that dulling that alarm erased the frustration without improving recall. Bilingualism, caffeine and age all shift how often it happens, though the sources stress these states are independent of dementia.

• A study of 51 languages found 45 describe the feeling with the tongue, mouth or throat, while American Sign Language users report a tip of the finger state
• The blocking hypothesis, where wrong words like castanet crowd out catamaran, and the rival view that such words are a harmless byproduct
• Participants on lorazepam gave more wrong answers but felt none of the familiar agony
• 200 milligrams of caffeine reduced these states, until on-screen distractions made the caffeinated group perform worse
• Age-related gray matter loss in the left insula and how a single first-letter cue triggers spreading activation

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The oldest known impacted wisdom tooth belongs to a Magdalenian woman who lived in Ice Age Europe between 13,000 and 11,000 BCE. Third molars were grinding tools for raw roots, nuts and uncooked meat, and the heavy chewing load stimulated osteoblasts to build longer, bigger jaws. Agriculture about 10,000 years ago, cooking and then the Industrial Revolution softened diets, jaws grew shorter and wider while tooth size stayed the same, and impaction became ten times more common after industrialization. Genes such as PAX9 and MSX1 govern agenesis, the absence of wisdom teeth, which is almost unknown among Aboriginal Tasmanians and almost universal among indigenous people in Mexico.

Wisdom teeth usually erupt between 17 and 21, sometimes as early as 13, and Aristotle linked their arrival near age 20 to wisdom. About 12 percent of impacted wisdom teeth cause disease, including decay that spreads to the second molar, periodontitis, pericoronitis under a gum flap called the operculum, and odontogenic cysts that can quietly destroy bone. The 1970s idea that wisdom teeth push front teeth into crowding has been debunked, and the UK's NICE opposes removing healthy impacted teeth. Data from 2024 show 53 percent of Americans have had them removed, but only 26 percent of those aged 18 to 29.

• How chewing tough food signals bone to grow, and why a soft diet left too little room
• Pericoronitis pain that can radiate to the ear and throat and cause trismus, locking the jaw
• Odontogenic cysts, found in up to 2.2 percent of impacted teeth, growing over 2 to 13 years
• Inferior alveolar nerve damage in about 1 in 300 extractions, and the risk of dry socket
• Platelet rich fibrin plugs spun from the patient's own blood to reduce pain, swelling and dry socket

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Tickling is two different things. In 1897 psychologists G. Stanley Hall and Arthur Allen separated knismesis, a light moving itch that annoys without laughter and is shared with animals like scratching dogs and twitching horses, from gargalesis, the harder pressure that brings helpless laughter and is found mostly in humans and other primates, though rats emit ultrasonic chirps when tickled. In 1939 Yngve Zotterman at the Karolinska Institute showed in cats that tickle depends on the same nerves that carry pain. Cutting pain nerves to treat chronic pain dulls the tickle response, and German brain studies link gargalesis to defensive circuits in the hypothalamus. The soles of the feet top the list of ticklish spots, then armpits, neck, under the chin and ribs, while the nerve rich palms do not.

Darwin argued tickling needs the anticipation of pleasure, since a stranger's tickle brings fear. In a survey of 84 college students, only 32 percent liked being tickled and 36 percent disliked it, yet some who disliked it smiled more than those who enjoyed it. Galileo, in The Assayer, used tickling to argue sensations exist in the perceiver, and Francis Bacon noted that grieving men still laugh when tickled. The combat theory, from J.C. Gregory in 1924 and later psychiatrist Donald W. Black, notes that ticklish spots match the body's most vulnerable areas in a fight.

• Christenfeld and Harris's blindfolded tickle machine, which drew as much laughter as a person
• Aristotle's puzzle about self tickling and Blakemore's 1998 brain imaging showing the cerebellum cancels the sensation
• Why some people with schizophrenia or related traits can tickle themselves
• Heinz Heger's account of guards at Flossenbürg torturing a prisoner with goose feathers, and the Japanese punishment kusuguri-zeme
• Vernon Wiehe's research with 150 adults abused by siblings, including forced tickling that caused vomiting and loss of consciousness

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In Catholic and Eastern Orthodox tradition, incorruptibility is the belief that divine intervention lets the bodies of some saints and beatified people avoid normal decay through autolysis and putrefaction. The Catholic Church verifies claims under canon law: bishop authorized inspections, sometimes a pontifical commission, a written medical report, and resealing with ribbons and wax. Witnesses often report a sweet odor of sanctity, and embalming or organ removal disqualifies a body. When Pope John XXIII was found intact, officials pointed to his embalming and his sealed triple coffin, where oxygen loss starves aerobic decay bacteria.

Bernadette Soubirous of Lourdes was inspected in 1919 and 1925 and now lies with wax coverings on her face and hands, and John Vianney is shown under a wax mask, partly because preserved tissue darkens and sinks or turns to waxy adipocere. Teresa of Ávila's remains were judged unchanged in 1750, 1914 and again in 2024, while Bonaventure found only the tongue and jaw of preacher Anthony of Padua intact in 1263. Incorruptibility is no longer a required miracle for sainthood. The Tablet has reported repairs with wax and silicone, and Monsignor Robert Sarno has called the belief false, citing cold, oxygen loss and soil. Louis Ginzberg's Legends of the Jews records similar claims about the scribe Baruch, whose tomb is in Iraq.

• How canon law turns an exhumation into something close to a forensic investigation
• Saponification and grave wax, and why wax masks are applied for appearance
• Peat bog bodies as evidence that acidic soil can preserve skin like leather
• Alexander of Svir's relics, seized by the Bolsheviks in 1918, kept at Petrograd's Military Medical Academy and recovered in 1997
• Joasaph of Belgorod's body, taken in 1918, found in Leningrad's Museum of Religion and Atheism and returned in 1991

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Most people see color through three cone types tuned to short, medium and long wavelengths, but the common ancestor of all vertebrates had four. During the nocturnal bottleneck, when early mammals hid from dinosaurs, rods mattered more than cones and mammals lost two cone types, and primates later re-evolved a third, mainly to spot ripe fruit. Birds kept the fourth cone and see into the 300 to 400 nanometer ultraviolet range, so zebra finches that look alike to us carry distinct UV plumage patterns, and pigmented oil droplets in their photoreceptors sharpen each color channel. Daytime raptors shifted that cone toward violet, trading UV for sharp focus because short wavelengths cause chromatic aberration.

Humans can rebuild the fourth cone through the X chromosome, where the red and green opsin genes sit side by side. A woman who inherits a variant opsin from a father with mild color blindness can, through random X inactivation, develop a mosaic retina with four photopigments, and estimates of such carriers run from 15 to 50 percent of women. Most remain functionally trichromatic because the brain lacks a fourth opponent channel. Neuroscientist Gabriele Jordan searched for 20 years before subject cDa29, in 2010, reliably told pure orange light from a red green mixture in a Rayleigh match.

• Mice engineered with a human cone pigment that gained measurable color discrimination
• Conditional tetrachromacy in twilight, when rods peaking near 500 nanometers feed into color processing
• Aphakia, where people whose lenses were removed in cataract surgery report seeing ultraviolet as a ghostly whitish blue or violet
• Pentachromacy in some lampreys and pigeons and 15 opsin classes in the bluebottle butterfly
• The mantis shrimp and its 33 distinct light detecting proteins

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Up to half of all people will wake at least once unable to move, with crushing pressure on the chest and a dark figure in the room. This episode explains sleep paralysis as an overlap of REM sleep and waking: serotonin, the wake signal, and acetylcholine, which drives REM, fire at the same time, so the mind is alert while REM atonia keeps the body locked. Dream activity bleeds into perception as static, buzzing, whispers, roars and electric tingles. The midbrain's threat vigilance system reacts to the helplessness, and because REM breathing is shallow and cannot be deepened at will, the brain reads that resistance as something sitting on the chest. Vestibular glitches add floating, spinning and out of body sensations.

Cultures have named the intruder for centuries. The word nightmare comes from the Old English mare, a goblin that sat on sleepers' chests. In Newfoundland the Old Hag was fought with a hag board, a shingle studded with upturned nails strapped to the chest, and in Italy the Pandafeche witch was distracted by a bag of sand or beans she must count. Japan calls it kanashibari, Cambodia says the ghost pushes you down, Scrooge blamed Marley on an undigested bit of beef, and many Americans describe alien abductions. A comparison of Egypt, where episodes are attributed to the jinn, with Denmark found episodes three times as frequent and longer in Egypt.

• Why the brain invents an intruder to explain paralysis and restricted breathing
• Half of Egyptian sufferers fear dying of suffocation during an episode, versus 17 percent in Denmark
• Sleeping on the back, sleep deprivation and shift work as the triggers researchers cite
• The difference between isolated sleep paralysis on waking and the episodes at sleep onset seen in narcolepsy
• CBT-ISP, which teaches diaphragmatic breathing and reframing, and the use of tricyclics or SSRIs in severe cases

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In 1953 Nathaniel Kleitman and Eugene Aserinsky noticed sleepers' eyes darting beneath closed lids and found that people woken in those windows reported vivid dreams. EEG later showed REM sleep looks like wakefulness: fast, desynchronized signals, theta rhythms in the hippocampus and gamma waves of 40 to 60 hertz. Acetylcholine floods the brain while histamine, serotonin and norepinephrine drop to zero, and Hobson and McCarley described brainstem REM-on and REM-off neurons switching the state. To keep the body still, the medulla hyperpolarizes motor neurons by 2 to 10 millivolts, producing paralysis below the neck. When that fails, REM behavior disorder lets people act out dreams, and in narcolepsy the switch intrudes into waking life as cataplexy. REM also suspends temperature control, so sleepers stop shivering, and cold enough rooms block REM entirely.

PGO waves fire from the brainstem into the visual cortex about every six seconds and drive the eye movements, while emotional regions light up and the prefrontal cortex dims. People woken from REM solve anagrams better, and SSRIs suppress REM until a vivid rebound. REM-like states appear in bearded dragons and jumping spiders, predators get more than prey, and newborns spend 80 percent of sleep in REM. Explanations range from memory consolidation and Crick and Mitchison's unlearning of parasitic modes to tonic immobility and oxygenating the cornea.

• Jumping spiders whose legs curl during REM-like bouts, a sign of lost internal fluid pressure
• The ontogenetic hypothesis that REM wires the developing brain before birth
• Rats on flower pot platforms in water that fell in whenever REM paralysis began and died within four to six weeks
• The paradox that REM deprivation temporarily eases depressive symptoms
• Why humans on long term SSRIs survive suppressed REM while deprived rats do not