ELI5: When sending a letter abroad, how does the receiving country's mail service get paid for their work?

There is a global unified postal service body called the Universal Postal Union . The Union creates a framework and standards so that ALL countries can exchange mail freely without forming individual contracts with every other country.

When one pays for the postage for international mail, a portion of the money goes to the home country’s postal service and a portion goes to the destination’s postal service. The portion of money exchanged depends on how much mail (in weight) each country is exchanging. This is set up so that the destination country receives money for delivering the postage. Countries that receive more mail annually get less money per kilogram of mail.

The fee paid to the destination county is called a terminal due.

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ELI5: why do you see weird patterns when you close your eyes and apply pressure onto them?

This phenomenon is called phosphene.

When light hits the cells in the eye, these cells send a signal to the brain to give an image of what is seen. These cells are called photoreceptor cells, and their main means of activation is when a photon of light hits them. Another way to activate them is via mechanical stimulation (aka applying pressure to they eyes). When you apply mechanical stimulation, the subsequent activation of the cells will be random (not patterned), and when this signal is transmitted to the brain areas that are responsible for generating an image, you will see weird patterns instead of the normal images that would be generated by photon-induced stimulation.

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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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