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Why anesthesia makes us pass out -

Used in the US since 1846, it's still unclear why general anesthesia causes people to pass out. A recent study showed that it affects proteins in the brain, and the reason we go unconscious has to do with altering neural activity. However, more research is needed.

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Blushing -

Blushing is often connected with shyness or embarrassment. Some experts believe it evolved to show submission to group leaders. But it's still a mystery.

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Crying -

Physiologically, it's understood what's happening when someone cries. But the evolutionary reason is still a scientific mystery. It's known that babies cry to communicate and get attention. Some experts believe that for adults it might be for social reasons, like to bond or to show when something isn't right.

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Laughing -

It's still not known why people evolved the ability to laugh. But experts believe it has something to do with communication, not with humor. It's also possible that laughter is to let other people know that we’re okay or to bond with each other.

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How medications like Tylenol work -

It's still not entirely understood why pain relievers containing acetaminophen give us pain relief. It's known that acetaminophens aren’t totally consistent, for example they're more effective in some types of cells than in others. Therefore, scientists believe the drug might be a specific type of enzyme inhibitor.

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Why we get hiccups or how to stop them -

While a lot of people have their favorite techniques to get rid of them, scientists actually don't know what causes hiccups, their purpose, or how to cure them.

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Why cats purr -

Scientists know how cats purr, but it's still not understood why they do it. Some suggest that it's caused by the constriction and dilation of the larynx, creating a vibration that makes the classic purring sound. But that's still just speculation.

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Why bicycles are able to stay up on their own -

Ever notice that a bike stays up on its own after a push? It's still not known how it manages to balance itself while still moving.

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Kissing -

Unlike intercourse and hugging, which make sense from an evolutionary standpoint, kissing is an act that still leaves scientists wondering. From releasing endorphins to connecting kissing to our primitive ancestors who fed their children via the mouth, there are many theories as to why we exchange saliva to show our affection.

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How skates work on ice -

A mystery of physics, there is actually one popular theory. As ice has a very thin layer of liquid on it, skating on top of it might create more liquid because the friction causes melting. So in the end, the skate is changing the ice by creating a path on which to glide on.

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Sleeping -

While there are theories as to why we need sleep, no one knows for sure. What is known is that it helps us recover from the day, and there’s evidence it changes the connections in our brains.

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Dreaming -

There are also no clear answers as to why we dream. Some experts believe dreaming doesn't have a purpose at all. Others think it's a reflection of our fears and experiences.

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Feeling pain -

From minor pain like a paper cut to moderate and chronic pain, most humans will experience some degree of physical pain in their life. However, science still can't explain why we feel pain and why there are different types of pain.

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Gravity -

If you thought Newton had sorted this question out, then you're wrong! While gravity is both incredibly weak and incredibly strong, there's still no answer as to why it can hold the entire universe together and be weaker than the force that brings two magnets together.

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Cure for aging -

While scientists know some things about why we age, it's still not fully understood. In the end, aging is probably the result of a complex group of poorly understood processes, meaning there isn't any cure happening anytime soon!

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Why humpback whales went from being solitary creatures to living in groups -

Humpback whales used to be a solitary creature, but since a few years back they began to appear in packs of 20 to 200 off the coast of South Africa. Marine scientists aren't sure why this changed.

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A cure for the common cold -

Colds actually come from seven separate families of viruses, and each of those have sub-viruses. So if there was a cure for a cold, it would need to be a cure that catches about 200 sub-viruses.

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Why the placebo effect happens -

Even if it has been proven again and again that treatments can actually make someone feel better, the placebo effect is still a scientific mystery. It’s believed that placebos somehow help release endorphins, but more information is still needed.

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Why it takes more genes to make a tomato than a human -

Tomatoes have over 30,000 genes, which is around 10,000 more than a human. The question that still baffles scientists is why tomatoes require more genes than the infinitely more complex human...

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Mood swings -

From going through puberty to being on your period, or simply having a bad day, most people have experienced mood swings. However, hormones might not be the only explanation. Links have also been made to an over-consumption of sugar and caffeine, which might explain mood swings that happen in everyday life.

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Why tornadoes start -

It's still not known why some thunderstorms create tornadoes and others don't. What's understood is that tornadoes develop when cold, dry air interacts with warm, humid air. But thunderstorms that develop from those conditions don't always cause tornadoes.

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Why tornadoes end -

It's also unclear why tornadoes end. However, some experts speculate that it has to do with the tornado’s interaction with cold temperatures.

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The urge to scratch -

It's often understood why we itch, but not why we have the urge to scratch. The body's receptors for itches are almost identical to those that convey pain, and scratching is believed to interfere with those signals.

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Having memories -

The act of forming and holding on to memories has always puzzled scientists, and continues to do so. How come we can remember big and small memories that occur throughout our life? And why can memories be altered each time we remember them? These are questions still being speculated on.

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Yawning -

You probably yawn every day, but, strangely, scientists still don't know why. One idea is that yawning is a thermoregulatory behavior that cools down the brain. But this is still unclear.

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Consciousness -

Frequently defined as how we feel present and alive in the world, it's still not known exactly what consciousness is. An interest in both philosophy and science, scientists would like to know which part of the brain is responsible for consciousness.

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Why glass becomes solid -

When holding a glass, it's pretty obvious that it's solid. However, unlike other solid objects, it doesn't have a well-ordered molecular structure. But that structure isn't irregular enough for it to qualify as a liquid. This makes it an unusual solid, which remains a mystery for scientists.

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The “nature vs. nurture” debate -

The “nature vs. nurture” debate still hasn't been settled. While we know that our genes interact with our environment to foster characteristics, it's still not sure to what extent.

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Why only some birds migrate -

It's still unclear to ornithologists why some birds migrate while others don't. The birds that do migrate probably do so to conserve energy in a place with plenty of food available.

Sources: (Mental Floss) (Insider) (BBC)

See also: Science still isn't able to explain these world mysteries 

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