Stargazers who live in major cities know all about light pollution, but this lack of complete darkness goes beyond geography. It turns out the sky is never totally black no matter where you live, even at night. We can mostly chalk this up to the luminous effects of moonlight and starlight, as well as to our atmosphere, which, in addition to making the planet’s air breathable, scatters all the light that passes through it. (This is what makes the sky appear blue during the day, as the atmosphere scatters blue light more than other colors.)

Water is intrinsically blue.

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Though it appears clear and colorless in small amounts, water naturally has a slight blue color caused by the absorption of light at red wavelengths.

In highly populated areas, the effect of light pollution is known as skyglow, and it’s what gives the night sky its milky, sometimes yellowish complexion. But even if you find yourself in an extremely remote location, the sky would be closer to a navy blue than black. As previously mentioned, this is partially because of the moon, but it’s also because the atmosphere itself emits a faint light of its own (in addition to the light it scatters), known as both airglow and nightglow. Depending on where you are, that glow could be any combination of red, green, purple, and/or yellow. If you’d like to see a completely black sky, you’d have to go to the moon, which has no atmosphere and thus nothing to scatter or emit light.

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It’s physically impossible to hum while pinching your nose. Go ahead, give it a try. See? To understand why humming, which we often associate with the mouth, would be affected by the nose, we have to look at the anatomy of our vocal folds. Vocal folds, aka vocal cords, are delicate bands of muscle located in the larynx, aka the voice box. When relaxed (i.e., when we’re quiet), our vocal folds resemble a wishbone. But when we speak, sing, yell, grunt, whisper — and, yes, hum — we send air up from the lungs and through the voice box. Simultaneously, those thin bands of muscle contract together, as if the open end of the wishbone has snapped shut. In reality, our vocal folds move in a wavelike pattern, vibrating against one another in varying frequencies that allow us to speak, sing, shout, and murmur. The faster our folds vibrate together, the higher the pitch; the slower, the lower.

We have “true” vocal cords and “false” vocal cords.

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“False” vocal cords, also known as the vestibular or ventricular folds, are thick folds of mucous membrane that sit above the “true” vocal folds to serve as backup protection against food or liquid entering the airway. Manipulating the “false” vocal cords can produce a deep, gravelly tone.

While we might think of humming as a sound that emanates from our mouth and lips, we actually produce this sound by sending air from the voice box to the nasal cavity and out through the nostrils. Thus, when we pinch our noses, there’s nowhere for the air to travel, and the vibration and corresponding sound stops. This also explains why pinching your nose mid-hum might make you feel like something’s caught in the back of your throat. (That’s the air trying to find a way out.) Another good way to test this theory is to hum when you’re congested. Depending on how blocked your nostrils are, you might not be able to hum at all, or you might even feel the mucus in your nose trying to move around as the air attempts to escape your nostrils — gross, but pretty interesting, too!

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Your eyes are how you see the world, but they’re also how you see yourself — including inside of your own eye. You may have experienced this while having your eyes examined, such as when the doctor shines a light on your pupils. When light comes from a small point and hits your eye just right, you can see your own blood cells as they move through the capillaries on your retina. The light reflects off the vessels, casting a shadow onto the light-sensitive cells in the retina and rendering them visible to your brain in a pattern of orange or red coloration. Each heartbeat sends those blood cells surging, adding a rhythmic quality to this strange phenomenon. 

The eye muscle is the fastest in the body.

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The orbicularis oculi, the muscle that controls the eyelids, is indeed faster than any other muscle. Each eye contains one, which closes and opens the eye in less than 100 milliseconds.

You may also have experienced what’s called the blue field entoptic phenomenon while staring at the clear blue sky and noticing clear, floating cells moving through your field of vision. Unlike red blood cells, white blood cells don’t absorb the short wavelength of the blue light, allowing you to see them as they flow through the blood vessels in front of the retina. These are different from floaters (small spots that move through your field of vision), which tend to increase with age as the vitreous fluid in your eye changes thickness. In most cases, floaters are a normal aspect of aging, though in excess they could be a sign that you should have your eyes checked.

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Gravity is an essential force on Earth: It keeps the planet in orbit at a safe and comfortable distance from the sun and even holds our atmosphere in place. It does have a downside, however: It weighs down the human body, making us a tiny bit shorter by the end of the day. From the moment we climb out of bed in the morning, gravitational forces push down on us, applying downward pressure on our joints, compressing our spines, and causing our organs to settle. All that strain adds up, enough to shrink a body by 1 centimeter. Gravity is at work whether we’re sitting or standing, but at bedtime, our bodies get a slight reprieve as lying down redirects the force. Sleeping horizontally gives our spines and joints time to decompress and gain back the height lost during the day, making us once again slightly taller by morning. 

Children have more vertebrae than adults.

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At birth, humans tend to have 33 vertebrae — the individual bones that make up the spinal column. By adulthood, many of those bones have completed the normal process of fusing together, leaving the average adult with just 24 vertebrae.

Even though this ebb and flow is a normal process our bodies endure, over time humans tend to shrink in stature. The human body constantly breaks down and replenishes its bones to keep them healthy, though by age 35, that process tends to slow, with bones breaking down faster than they rebuild. The aging process also causes the cushions between our bones to retain less water and deteriorate, which makes the bones settle together. These processes slowly chip away a few millimeters of height at a time, which can eventually add up — on average, men tend to lose an inch of height between 30 and 70 years old, while women can lose up to 2 inches.

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