Fire seems intrinsically linked to life on Earth. The fires of the Earth’s molten core formed the land we live on, forests are at their healthiest when they burn in a controlled manner to make way for new life, and even the legendary R&B group Earth, Wind & Fire paid homage to the stuff. In fact, Earth is the only place we know of where fire occurs. That’s because fire requires three things to exist: heat, oxygen, and combustible material. This “fire triangle” is only possible on Earth, as far as we know, because of the planet’s high levels of free oxygen. Travel to other planets and moons in the solar system, and there isn’t enough (or any) oxygen for fire to exist. As for the sun, which some people imagine as a giant ball of fire, it’s actually a giant collection of gas that glows thanks to the complex nuclear fusion occurring in its core. 

Humans discovered fire 20,000 years ago.

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Fire has been a constant companion for Homo sapiens. Scientists estimate that widespread fire use is generally found in the fossil record some 200,000 years ago, but evidence from a few archaeological sites pushes that back to 500,000 years or more.

While fire may not exist in any other place in the solar system (as far as we’re aware), that doesn’t mean humans can’t export the stuff. NASA has experimented with fire in microgravity for more than a decade — both to discover its behavior and also design fire-resistant space material. Turns out fire in microgravity behaves much differently, appearing more sluggish (likely due to low oxygen) and forming strange, orb-like flames. So while it’s not impossible for fire to exist beyond the confines of Earth, it’ll need a lot of help from future astronauts to export it to the moon, Mars, and beyond.

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Time seems like a simple enough concept — 60 seconds in a minute, 60 minutes in an hour, and so on. That is, except for a little something called “gravitational time dilation.” First explored in Albert Einstein’s general theory of relativity, the idea is almost confusingly simple — the farther away you are from a massive object (e.g., a planet), the faster time travels. The more massive the object, the slower time travels, which is why things get very wonky around supermassive black holes like the one at the center of our galaxy. 

Albert Einstein first proposed the idea of time travel.

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The concept of time travel was popularized by early science fiction. The first mention of it (at least as a voyage controlled by technology) comes from H.G. Wells, whose 1895 novel “The Time Machine” was published years before the arrival of Einstein’s famous theory of relativity.

These differences in how time flows are minuscule on Earth, so they don’t really affect us — the top floor of the Burj Khalifa, the world’s tallest building, does not operate with a special time-dilated clock. Yet technically, even our heads experience time just a bit differently than our feet. In 2010, the U.S. National Standards and Technology (NIST) even performed an experiment using optical atomic clocks that could measure a change in time dilation within less than 1 meter. 

Although imperceptible to our minds, precision technologies such as GPS need to factor in time dilation in order to work at all. So the next time you use Google Maps, consider giving a shout-out to Einstein and his mind-bending theory of the universe. 

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Smell is one of humanity’s most important, and often overlooked, senses. It’s intimately tied to taste and memory, and plays a pivotal role in detecting danger, whether from fires or rotten food. It may even play a role in how we choose our mates.

One little-known aspect of smell is how it fluctuates throughout the day. According to research conducted by Brown University and published in the journal Chemical Senses in 2017, our sense of smell is somewhat regulated by our circadian rhythm, the internal biological process that regulates a human’s wake-sleep cycle. (If you’ve ever traveled across the ocean, the resulting jet lag is a disruption of this rhythm.)

Smell in humans begins declining after the age of 30.

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When we’re born, humans can only sense certain smells, such as a mother’s body. However, our sense of smell really takes off at the age of 8, and is usually stable until around age 50. After that, our nose powers decline, and drop off precipitously after the age of 70.

The Brown study analyzed 37 teenagers for a week, and measured their sense of smell against their levels of the sleep-inducing hormone melatonin. A rise in melatonin meant that the body’s nighttime circadian rhythm was kicking in, essentially saying, “It’s time to sleep.” The results showed that the teens’ sense of smell was at its highest in the evening, around 9 p.m., or what the researchers called the beginning of “biological night.” Conversely, their sense of smell was at its lowest between the hours of 3 a.m. and 9 a.m., when the body has little need for sniffing. Scientists can only guess at why the body kicks its olfactory receptors into high gear at 9 p.m. — it may help humans ensure satiety following the last meal of the day, scan for nearby threats before sleeping, or act as a means for encouraging that aforementioned mate choice.

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