If you’ve ever taken an important test such as the SAT, you know just how valuable a pencil eraser can be. Without one, you’d be stuck with an answer you quickly realized was incorrect, or perhaps a multiple choice bubble you’ve filled in by mistake.

It’s easy to take the little nub on the end of your pencil for granted until you need it and realize it’s broken off (or perhaps been chewed off) at some point. But have you ever stopped to wonder how it works? Unlike the delete key on your computer, it’s not a simple “opposite” action undoing the first. The rubber on one end of a pencil is a completely different material than the graphite of the tip. 

Why would one remove the other, making pencil strokes and smudges vanish like they were never there? Here’s the science behind erasing pencil — and pen — marks.

Mistakes Happen

Let’s start with pencils. As you may know, modern pencils aren’t actually made with lead, but rather with graphite, a soft form of carbon, mixed with clay. When you write, microscopic flakes of graphite chip off from the tip and get trapped within the paper’s fibers, leaving the marks that you see.

The graphite flakes stay on the paper because of a chemical attraction called van der Waals force. Think of it as the “glue” of the universe, binding molecules together thanks to positive and negative charges that attract each other.

The bond that graphite particles make with the paper isn’t very strong, which is helpful when you need to erase a mistake. As you rub your eraser across the paper, the mechanical friction upsets the atomic attraction between the graphite and paper, and those pencil marks end up sticking to your eraser. Basically, the graphite switches partners midway through a dance, leaving the paper for the eraser.

Rubber makes a great eraser material for a couple of reasons. For one, it’s soft and nonabrasive, so you can safely rub it against your paper, unlike using, say, steel wool. What’s more, rubber and graphite are both nonpolar, or neutral — so they naturally attract. This makes it easy for pencil marks (and your mistakes) to not only escape the paper but also adhere to the eraser shavings, which can then be quickly brushed away.

Gum erasers are also made of rubber, but they’re even softer on paper (though more delicate and shorter-lasting), which is why they’re often used by artists. Kneaded erasers are made of even softer, synthetic rubber, and are often used to remove charcoal in addition to pencil marks.

Instead of breaking off, a kneaded eraser holds what it removes in one piece, like a glob of Silly Putty. These erasers need to be cleaned, but they don’t leave behind a mess of eraser shavings. Vinyl and plastic erasers are also available — they erase very cleanly, but can be tough on paper.

Can You Erase Pens?

EraserMate, one of the first commercial erasable pens, came out in the late 1970s. It wasn’t just an eraser tacked onto a pen, though — to make the pen erasable, the ink needed to be different. It incorporated rubber cement, which made it work similarly to the graphite in pencils, keeping the ink from absorbing into the paper. Because the ink just sat on top, it could be rubbed off with an eraser.

Modern erasable pens work quite differently. Most now use thermochromic ink, which becomes invisible when it gets hot. Friction causes heat (think: rubbing two sticks together to start a fire), so rubbing an eraser over thermochromic ink heats it up, essentially making the words “disappear.” 

Because it’s invisible and not gone, you can try making the ink reappear by sticking your paper in the freezer. Apparently, some mistakes can never be undone.

Who can resist approaching a parrot and saying, “Polly, want a cracker?” Humanity has been fascinated by these chatty birds for millennia — the earliest evidence of captive parrots dates to a 5,000-year-old cave painting in Brazil. These colorful avians come in a variety of shapes, sizes, and shades, but we relate to them because they seem to relate to us through our own language. 

But why are parrots able to talk while our closest relatives, gorillas and chimps, are relegated to sign language? Because these birds are not so bird-brained. 

Birds of a Feather Talk Together

Parrots’ knack for communication stems from their highly social nature. Wild parrots live in flocks, where they use a complex system of vocalizations to coordinate flight, find food, and warn each other about danger. By using unique brain nuclei dedicated to advanced vocal learning, chicks learn to communicate in the nest by imitating mom and dad — just as human babies do.

Some researchers suggest that certain parrot species use “signature contact calls” to identify each other, in the way that humans use names. This may have evolved because most species are monogamous. You know the story. Two parrots meet. Fall in love. Raise a couple of chicks together. 

But unlike many of their human counterparts, parrots mate for life. And given the fact that parrots are long-lived — some species reach their 50s and beyond — it’s no wonder that parrot couples tend to develop a shorthand by mimicking each other, just like spouses who seem to invent a shared language. And this naturally evolved skill seems to have translated from the wild to captivity. 

Pet parrots mimic their owners to bond with them. But how does an animal with no lips and a rigid beak exclaim, “What’s up, Doc?” when you walk into the room? Parrots (and other birds that mimic humans, such as certain corvids, starlings, and mynahs) seem to defy anatomical physics with their linguistic talents. Our closest monkey relatives likely possess the physical physiology to speak human language, but lack the brain power to do so (don’t tell King Louie). 

The avian talent for mimicry is enabled by a special organ called the syrinx, which is similar to the human larynx. The syrinx is a highly effective voice box that allows birds to make a wide variety of sounds, from singing to quacking, screeching, and trilling. Couple that syrinx with a bird’s advanced vocal learning abilities, and the next thing you know, Polly’s barking like a dog to taunt your cat.

Of all the birds that can mimic human speech, parrots are best at it thanks to their unique intelligence. They possess a highly developed region called the medial spiriform nucleus, which may act as a superhighway between the cortex and cerebellum. This pathway is similar to the pontine nuclei, a neural circuit in primates related to motor skills and intelligence. 

Parrots are as brilliant as they are beautiful (have you ever seen a Rainbow Lorikeet?), but do they actually know what they’re saying? Historically, scientists have said no. But one particularly intelligent African grey named Alex could associate words with objects and other concepts. 

Alex (short for “avian language experiment”) understood more than 100 words, could count to six, and was able to use simple adjectives such as colors and “smaller” or “bigger” to describe things. 

So if you ask Polly if they want a cracker and they say yes, you better have some crackers on you! 

There’s a myth that suggests hanging from a bar can make you taller by decompressing your spine. While there are many benefits to this “dead hang” exercise, including improved posture and grip strength, any height gains are temporary.

Yet there is one group of people who grow a couple inches every time they go into their unusual office: astronauts. Here’s what zero gravity does to their spines.

To New Heights

In space, astronauts experience microgravity (Earth is still pulling on them a little bit), a condition in which they seem to be weightless. Everything they do must be modified to account for the fact that without gravity, they float. 

Aboard the International Space Station, crewmembers sleep in pods with sleeping bags tied to the wall, and navigate by using handrails and gently pushing off walls. To work, they must strap themselves in or they’ll drift away.

That same lack of gravity also changes the human body. With no force pressing down on the spine, it decompresses when the spongy discs between the vertebrae expand and relax. Picture your spine, which is essentially a string of alternating bone and cartilage, stretching out like an accordion (though not nearly as dramatically). 

This results in a height bump of about 3% over the first 3 to 4 days spent in space. NASA astronaut Kate Rubins measured 5 feet, 6 inches tall on Earth, but grew to just over 5 feet, 7 inches tall in space. An astronaut who was 6 feet tall would gain about 2 inches. 

Because most of this increase is concentrated in the spinal column, the astronaut’s standing and sitting height changes, as well as their shoulder height. Their arms are farther from the ground, letting them reach objects positioned higher up. 

But just like dangling from a bar, this change doesn’t last forever. Once they return home, astronauts’ spines compress and bring them back to their “Earth” heights.

Growing Pains

While it might seem like becoming temporarily taller is a fun perk of going to space, microgravity can be harmful to the human body. When astronauts don’t have to bear weight while standing, sitting, or walking, they lose muscle mass. Their bones weaken, losing an average of 1% to 1.5% of their mineral density for every month in space. Bodily fluids also shift up and into the head, putting pressure on the eyes. 

About 70% of astronauts report back pain after a few days in space, and roughly half experience it back on Earth. One study conducted MRI scans of six NASA astronauts’ spines before their missions, immediately after they returned to Earth, and again one to two months later. The astronauts had all spent four to seven months in microgravity aboard the ISS. 

All six had experienced atrophy of the lean, functional muscle that supports the spine, and recovered only two-thirds of that loss one to two months after returning to Earth. They also faced four times the normal risk of a herniated disc. 

This is why NASA and other space agencies continue to study and find ways to counter the effects of microgravity on the human body. For example, compression cuffs can help keep blood in lower extremities, similar to wearing compression socks on long flights. 

Specific exercises can also help maintain bone and muscle strength. This is especially important as humans consider even longer missions and extended stays in space, and if we ever hope to reach Mars and other more distant destinations.

After all, we don’t want to take any small steps or giant leaps if it means throwing out our backs.

You’re walking with a friend after a summer shower when they point excitedly toward the sky and exclaim: “Look at that rainbow!” The next moment, it feels like you’re both admiring the same spectacular ribbon of color. But here’s the twist: You aren’t. In a very real sense, each of you is looking at a different rainbow.

That sounds impossible. After all, the rainbow is right there, stretching across the sky. But unlike, say, a mountain or a cloud, a rainbow isn’t actually an object sitting in one particular place. It’s an optical phenomenon — something your brain creates when sunlight, raindrops, and your own position on Earth line up in just the right arrangement. 

Take a few steps to the left or right and the rainbow changes with you. That makes every rainbow you see uniquely yours. So what exactly are you looking at?

For Your Eyes Only

If you were walking toward a shallow pool and kept the same pace once you entered it, you’d still slow down and move differently. Well, light travels differently through water as well.

Rainbows begin when sunlight enters millions of tiny water droplets suspended in the air, often after rain. As the light enters each droplet, it slows and bends, because water is denser than air. This process is called refraction. Sunlight then reflects off the back of the droplet before bending again as it leaves the way it came. 

Because different colors of light have different wavelengths, they bend by slightly different amounts. Sunlight disperses into its familiar spectrum of red (which has the longest wavelength), orange, yellow, green, blue, and violet (which has the shortest wavelength).

But here’s the part that makes rainbows so personal. Only droplets positioned at just the right angle send rainbow light toward your eyes. For the most familiar-looking rainbows, that angle is about 42 degrees from the point in the sky directly opposite the sun, known as the antisolar point.

Imagine an invisible line running from the sun behind you, through your head, and out into the sky. That line is unique to you, and it’s key to what creates the rainbow you see. Since your friend is standing in a slightly different place, even if it’s only a couple of feet away, their invisible line is different (unless they’re really invading your personal space). That means a different collection of raindrops in the sky is directing colored light into their eyes. So although your rainbows look nearly identical, they’re created from entirely different droplets.

The effect is typically so subtle that you never notice it. But physically speaking, you and your friend are not seeing the same rainbow at all.

This also explains one of rainbows’ strangest qualities: They seem to follow you. If you walk forward, your rainbow doesn’t stay behind like a billboard or fire hydrant. Instead, a whole new grouping of raindrops takes over, bending light toward your latest position. The old droplets stop contributing, and countless new ones step in, as if doing “the wave” at a sports arena. To you, the rainbow appears to remain in place.

Even if you stand still, the droplets forming the rainbow are changing, as they’re often falling through the air the entire time. The rainbow is continuously rebuilding itself from moment to moment. If someone were standing exactly where you were a few seconds earlier, they wouldn’t see precisely the same rainbow as you did because the raindrops have already moved on.

This is also why it’s impossible to find the end of a rainbow, no matter how much you’d like a pot of gold. Sorry to break the news, but there isn’t one. Because a rainbow’s location isn’t fixed and moves with you, its “end” is always just out of reach

Then again, you’ve already found the real treasure. While rainbows may be among the most accessible wonders in nature, they’re also among the most personal. Gold’s got nothing on that.