ELI5: What exactly did John Oliver do in the latest episode of Last Week Tonight by forgiving $15 million in medical debt?

Whenever you take out a loan or get a service that you don’t pay for in advance, you owe someone money. If you don’t pay, they chase you down looking to get their money. For many of these people who are owed money, they want to spend their time performing services for people, not chasing down delinquent payers. So for a fraction of the total price of the debt, they sell the right to collect the money to some third party. That way they get some of their money back (more than they would without a bunch more effort chasing down the people who haven’t been paying).

So now the third party who specializes in chasing these people down will try to find the people who haven’t paid and get them to pay. What John Oliver did was buy the right to collect these debts, just like these third parties do, but then forgive the debts- tell the people who owe money that they don’t have to pay him back.

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ELI5: Why is menthol "cold"?

The people saying it’s because of evaporative cooling are wrong. Menthol’s boiling point is 212 Celsius, much warmer than your body.

Menthol isn’t really cold, it just tricks your body into thinking it is. There’s a type of nerve cell that responds to things like temperature, pressure, pH, etc. Some of these cells have what’s called a TRPM8 receptor on their surface. When menthol comes into contact with a TRPM8 receptor it binds to it, which makes the affected cell open an ion channel that admits sodium and calcium ions into the cell. This in turn causes the nerve cell to send a signal to the brain that the brain interprets as coldness. A similar receptor, TRPV1, is why the capsaicin in hot peppers feels ‘hot’.

Basically, menthol binds to a receptor on certain temperature-sensitive nerve cells, causing them to fire, and your brain interprets this nervous activity as coldness.

EDIT: Okay, evaporative cooling probably does have something to do with it, and it isn’t necessary for a substance to reach it’s boiling point to evaporate. However, I’m willing to bet that the cooling sensation is caused overwhelmingly by TRPV8 activation.

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ELI5: How do people solve rubik`s cubes in <10 seconds?

Rubik’s cubes are actually solved with a formula, or simple set of rules. Once you know those rules by heart, it just becomes a matter of applying them in the most efficient way (to minimize the number of moves required) and then moving your hands very, very quickly. Those with good memories for images can actually look at the cube once then put on a blindfold and solve it from memory.

Fun fact - every possible (valid) permutation of a Rubik’s Cube has a solution which is no larger than 20 moves. This is called “God’s Number” and it took about 30 years to determine this number. Over 99% of all possible permutations require 16-19 moves to solve.

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ELI5: When you go on a roller coaster or drive fast down a hill, why do you get that stomach drop feeling? What is actually happening inside your body that causes the feeling?

“The honest answer is that no one knows with absolute certainty,” said Dr. Brad Sagura, a surgeon at University of Minnesota’s Amplatz Children’s Hospital.

“There’s a cast network of nerve connections within the body, handling messages between the spinal cord, the brain and other structures,” Sagura said.

When you hit the peak of a roller coaster and start dropping so quickly, things inside start to shift around.

“The liver and spleen are relatively secured by suspensory ligaments,” Sagura said. “But the intestines themselves are relatively mobile. While your body is secured by your seat belt, the organs are free to move about by some extent. That contributes to the free-fall floating sensation that either calls us back for more, or has us running to get sick from nausea.”

The movement isn’t only the movement of the organs, it is also the movement of what’s inside the organs.

“The intestines, the stomach, they hold liquid,” Sagura said. “The bladder; the same thing. “It’s relatively fixed, but the fluid within those structures probably plays a role in that sudden drop.”

Sagura said there’s no long-term danger from your organs slightly shifting around. They go back to where they started. But the movement is enough for your nerves to notice that something’s happening, he said.

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ELI5: Why do pens dry out when the cap is left off, but the caps themselves have holes?

The pens dry out because the ink contains solvents. The solvents are volatile and will evaporate if exposed to the air too much.

The caps slow this down but they don’t completely prevent it. The solvents will still evaporate but will largely just sit inside the lid if there’s no air moving past the pen.

The holes in the lid are there so children are less likely to choke and suffocate if they swallow one.

The pen cap holes are also useful to pass a balloon catheter through the cap to aid the removal of the cap from the child.

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ELI5: Why do we recognize that mirrors are "silver" when they appear to be the exact same color as what they are reflecting?

Not all mirrors are perfectly reflective, and things can be done to a mirror to make it less reflective (like fogging, smudging, etc).

If you look at something like the side of a butter knife, you can see that it has a color, despite also reflecting the world around it. If you polish the side of that knife enough, eventually you’d get a usable mirror.

Common household mirrors are a thin layout of silver on the back of a pane of glass, so their color when less than perfectly reflective is the same as the color of unpolished silver.

As had been pointed out, modern mirrors use aluminum, since it is almost as good but a lot cheaper. Still, the language evolved in a time when aluminum was less common.

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ELI5:How does rabies make it's victims 'afraid' of water?

The virus affects the entire body, and especially hits us neurologically. What happens when you’re thirsty and you see water? You salivate in anticipation of relieving that thirst. Salivation leads to swallowing, lest we drool. Well for someone in the later stages of rabies, swallowing becomes a very painful act…and as with anything painful, the mind tends to not want to repeat the act that leads to the pain. The Rabies virus causes severe muscle spasms in the throat, and even the sight of water can set them off. If that were happening to you, wouldn’t you be ‘afraid’ of water, too?

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