<ul><li>The molecules in space are individually moving very fast, which means that each one has a lot of energy, and temperature is usually defined as the average kinetic energy of the molecules in a substance.</li><li>But there are so few molecules in space that even though each one has a lot of energy, the total amount of heat energy is small, which means it doesn’t warm things up very much.</li><li>It may be warm in theory, but it feels cold in practice.</li><li>Space may be hot, but it’s the hottest place you can freeze to death.</li></ul>

Is space hot or cold?

Could a person eat a whole cloud?<ul><li>No, unless you’re allowed to squeeze the air out first.</li><li>Clouds are made of water, which is edible. Or drinkable.</li><li>If you try to eat a cloud, you’ll just burp out more cloud faster than you can eat it.</li><li>A fluffy cloud the size of a house contains about a liter of liquid water, or 2 or 3 large glasses, which is about the volume a human stomach can hold at one time.</li></ul>

Eat A Cloud

What would be the most expensive way to fill a size 11 shoebox (e.g., with 64 GB Micro SD cards all full of legally purchased music)?<ul><li>If you really want to fill a shoebox with an arbitrarily large amount of money, you could get the US treasury to mint you a trillion-dollar platinum coin, something that—due to a loophole in a law around minting commemorative coins—it is technically authorized to do.</li></ul>

EXPENSIVE SHOEBOX

As plastic is made from oil and oil is made from dead dinosaurs, how much actual real dinosaur is there in a plastic dinosaur?<ul><li>Coal and oil are called fossil fuels because they formed over millions of years from the remains of dead organisms buried underground. </li><li>The standard answer to “What kind of dead stuff does the oil in the ground come from?” is “Marine plankton and algae.” In other words, there are no dinosaur fossils in those fossil fuels.</li><li>Land life, on the other hand, is more likely to form peat and eventually coal.</li></ul>

PLASTIC DINOSAURS

If I pulled out my eyeball and aimed it so that it was looking into my other eyeball, what would I see (assuming the nerves and veins remain undamaged)?<ul><li>Pointing an eyeball at an eyeball doesn’t create some kind of weird loop, like pointing a camera at its own video feed.</li><li>Each eyeball just sees an eyeball.</li></ul>

EYEBALL

If a T. rex were released in New York City, how many humans/day would it need to consume to get its needed calorie intake?<ul><li>About half of an adult, or one ten-year-old child.</li><li>Tyrannosaurus rex weighed about as much as an elephant.</li><li>T. rex ate seem to cluster around 40,000 calories per day.</li><li>According to Ryan’s shirt, an 80-kg human contains about 110,000 calories of energy, so a T. rex would need to consume a human every two days or so.</li></ul>

T. REX CALORIES

What if you were hanging on a helicopter blade by your hands and then someone turned it on?<ul><li>Helicopter rotors take a little while to get up to speed.</li><li>Once the rotor starts moving, it might take 10 or 15 seconds for it to make its first full turn, so you’d have enough amount of time to make eye contact with the pilot before you rotated out of view.</li><li>If you were hanging on, by that point you’d already be swinging noticeably outward and you’d feel an extra 10 or 20 pounds of weight from the centrifugal force.</li></ul>

HELICOPTER RIDE

Can you use a magnifying glass and the moonlight to light a fire?<ul><li>You can’t start a fire with moonlight, no matter how big your magnifying glass is.</li><li>You can’t use lenses and mirrors to make something hotter than the surface of the light source itself.</li><li>Lenses don’t concentrate light down on to a point—not unless the light source is also a point. They concentrate light down on to an area, creating a tiny image of the Sun.</li></ul>

MOONLIGHT FIRE

I noticed recently that the number of people in a family tree increases exponentially with each generation: Which got me thinking—are most people descended from the majority of Homo sapiens who have ever lived? If not, what fraction of all the people that have ever lived am I descended from?<ul><li>A 2004 simulation by Douglas L. T. Rohde and colleagues estimates that the identical ancestors point is likely somewhere between 5000 and 2000 BCE.</li><li>At that date, everyone who left descendants at all is an ancestor of everyone.</li></ul>

ANCESTOR FRACTION

I have a Nothing Grenade™. When detonated, it instantly replaces itself with a sphere of perfect vacuum 2 meters across. What would actually happen when it went off?<ul><li>The sphere of vacuum would collapse, colliding in the center with so much force that it would rapidly heat and might even turn briefly to plasma.</li><li>Energy would radiate outward both in the form of a pulse of heat and a shock wave, capable of causing severe injury or death and of destroying small structures. In other words, what you have is a regular grenade.</li></ul>

Nothing Grenade™

Would there be any danger from standing next to a large object that was 0 Kelvin?<ul><li>Cold objects can condense the air itself, causing liquid oxygen to collect on their surfaces like dew.</li><li>It’s highly reactive and tends to cause flammable things to spontaneously ignite.</li><li> A really cold object can set your house on fire.</li></ul>

DANGEROUSLY COLD

Why don’t compasses point toward the nearest hospital because of the magnetic fields created by MRI machines?<ul><li>MRI medical scanners have powerful magnets in them. The scanners are shielded, so the strongest parts of the magnetic field are contained inside the scanner, but a weaker field extends out around it. </li><li>While the magnetic fields from MRI machines aren’t strong enough to lure in compass-guided explorers from around the country.</li></ul>

MRI COMPASS

AN INSTANT NEW YORK TIMES BESTSELLER! An NPR Best Book of 2022 "The questions throughout What If? 2 are equal parts brilliant, gross, and wonderfully absurd and the answers are thorough, deeply researched, and great fun. . . . Science isn’t easy, but in Munroe’s capable hands, it surely can be fun." —TIME The #1 New York Times bestselling author of What If? and How To answers more of the weirdest questions you never thought to ask   The millions of people around the world who read and loved What If? still have questions, and those questions are getting stranger. Thank goodness xkcd creator Randall Munroe is here to help. Planning to ride a fire pole from the Moon back to Earth? The hardest part is sticking the landing. Hoping to cool the atmosphere by opening everyone’s freezer door at the same time? Maybe it’s time for a brief introduction to thermodynamics. Want to know what would happen if you rode a helicopter blade, built a billion-story building, made a lava lamp out of lava, or jumped on a geyser as it erupted? Okay, if you insist. Before you go on a cosmic road trip, feed the residents of New York City to a T. rex, or fill every church with bananas, be sure to consult this practical guide for impractical ideas. Unfazed by absurdity, Munroe consults the latest research on everything from swing-set physics to airliner catapult–design to answer his readers’ questions, clearly and concisely, with illuminating and occasionally terrifying illustrations. As he consistently demonstrates, you can learn a lot from examining how the world might work in very specific extreme circumstances.

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Randall Munroe, a former NASA roboticist, is the creator of the webcomic xkcd and the author of xkcd: volume 0. The International Astronomical Union recently named an asteroid after him; asteroid 4942 Munroe is big enough to cause a mass extinction if it ever hits a planet like Earth. He lives in Cambridge, Massachusetts.

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