To explain how the apps work, we need to understand how chips and contactless cards work.

Credit and debit cards are essentially just reusable check blanks. Written on them, you have the account number of the person trying to send money and the name of the account holder. When using the card to buy something at a shop, the payment computer has the account of the ones receiving the money (the shop) pre-programmed into it, and the employee at the till has punched in the amount to be sent. This is basically all the necessary components of a check. The payment computer phones up the bank with this info to request a transaction, and if the bank computer responds, "Looks great, we'll get that sorted!" the payment goes through and the terminal shows it as paid.

The magnetic strip on older cards is more or less just the info printed on the card, digitized, so the sale computer can read it quickly and mistake-free. Swiping a card is, for layman's purposes, hardly different than just punching in the data printed on the physical by hand, with a magic spellchecker that can tell if you typo'd it.

Now, if that's all a credit card actually was, just a name and account number, it'd be very easy to steal. So they have security built into them to make sure that only the rightful owner is using them. Basically, give the cardholder a test to prove it's actually them.

The oldest (and stupidest) form of this is the signature. The idea is that the card holder writes their signature on the physical card. Then, when making a sale, the cardholder also signs the receipt. The cashier should take the card and receipt, look at the two, and only allow the sale if the two match. The hope here is that A) your signature always looks the same every time you write it and B) only you can write it the way you do, no one could ever copy it. So if the signatures match, you must be the cardholder.

A much better solution is a PIN. It's basically just a tiny password that only the cardholder should know. If someone steals the card (or even just the numbers on the card), but do not know the tiny password, they can't use the card.

This idea is taken to the next level with a chip. In addition to giving you a PIN to memorize, the credit card company makes two identical copies of a tiny computer. One gets embedded into your card, and they keep the other. When you attach wires to this tiny computer and power it on (which is what inserting your chip into a chip reader actually does), you can send it some gibberish data, and it will answer back with more seemingly unrelated gibberish data. The key, though, is that every time you ask it the same gibberish question, it replies back with the same gibberish answer. So, when you insert the chip into a chip reader when making a sale, the card network can come up with a gibberish question, send it to your card's chip, and get the gibberish response back. It then asks the same gibberish question to the copy they have on hand. If the answers are the same, it must mean you have the chip, and by extension you must also have the physical card.

The beauty of the chip solution is that even if an eavesdropper somehow was listening to the conversation between the card company and the chip, and they overhear the gibberish question and answer, it's useless to them. That's because even if they technically know the "answer" to one of the gibberish questions, the card company will never ask that question again. All questions are single-use, and thus so are all answers. The only way to truly spoof the card is to be able to know every possible answer to every possible question. Or in other words, physically have the chip. So the chip effectively defends against people who know your numbers, but haven't physically stolen the card. A PIN is still required to defend against physical card theft.

Contactless tap is basically the same as chip, just done over radio waves instead of wires. This makes it easier to eavesdrop, but as we established already, eavesdropping on a chip payment isn't all that helpful to a thief, so we don't really care about that!

Now, finally, the payment apps. When you install a payment app and register a card to it, what you are essentially doing is turning your phone/watch/whatever into a credit card chip. The credit card company creates a secret program that works like a chip--takes a gibberish question in, gives a gibberish answer back--and installs a copy of it to your device. So when you tap your device to the reader, it gets asked a gibberish question, it creates a gibberish answer, and radios it back to the terminal, just like a chip. This proves that you have the physical device. It doesn't prove you have the physical card, but registering the card in the app in the first place did prove that you must have had it at some point, which is good enough.

When I worked at the DMV, one thing I learned very early when working with vehicle titles (i.e. deeds of ownership, but for vehicles) with multiple names on them is to pay careful attention to the conjunction between the names.

"Alice and Bob" is a different kind of ownership to "Alice or Bob". And meant that, to do anything with the title, you needed consent from everyone involved. Or, on the other hand, meant that any person on the title could do basically anything they wanted with it with only their own consent.

The conjunction was set up at the time the title was issued, and by filling out the application with a particular conjunction selected, you were giving your consent to play by the rules of that conjunction. So, by signing a form with an "or" conjunction between you and someone else, you are implicitly giving permission to the other person to give your consent for you. If that isn't what you want, that's what "and" is for.

Point being, in systems where robust rules are needed, they can be made. This is really just the basis of how all rules and agreements are created. The abstract concept of "ownership" is no different. It can be whatever you define it to mean.

If the test is written by a person, C is most likely. Because the answers are (probably) not 1/4 each.

If the test was shuffled by a machine, and the answers are perfect 1/4 chance, then no strategy is better than any other. Picking straight C is just as effective as picking C most of the time and picking B sometimes, and just as effective as picking with no pattern at all.

Let me ask a counter question:

You are playing a game where someone rolls a 6-sided die over and over, and each time you have to guess which number it lands on before it is rolled. If you guess correctly, you win.

You happen to know that the die is loaded. It's not a 1/6 chance for each number to show up. The side numbered "4" is slightly more likely to occur than all of the others.

Knowing that, why would you ever guess anything other than "4"? You're not going to win every time, but it's always your best shot.

As others have stated, if the exam has its answers distributed truly randomly (or at least sufficiently randomly, i.e. by a computer), and if all of the answers had their choice selection decided independently, then your guesses will not matter at all. You gain no statistical advantage by any strategy. You are simply rolling an X-sided die Y number of times, where X is how many choices each question has and Y is how many questions the exam has.

The adage that you should select the same letter multiple times in a row to get an edge stems from two things, one of which is completely unrelated (and may not apply) and the other only holds if the assumptions we made aren't true.

The first is about speed. If you mark every question with something, you are statistically expected to get at least a score of 1/X on all those questions you marked. So if you anticipate the possibility that you might not even finish your exam, if you have them all pre-marked (switching answers to the correct ones as you read your way through the exam for real), then you might get some extra scoring for your guesses on questions you may have otherwise marked blank. This only works, of course, if the exam you are taking doesn't penalize incorrect answers. An exam that marks non-answers and wrong answers the same benefits from guesses; an exam that subtracts points for wrong answers and does nothing for non-answers punishes guesses.

The second is that there is some evidence that in multiple-choice exams with answer keys arranged by humans one letter is statistically more likely to be the answer for any given question. In the common four-choice arrangement with A, B, C, and D, that letter tends to be C. So, provided your exam was written by a human, and the exam doesn't penalize guessing, answering all questions with C has a statistical advantage over random guessing.

DNA is the master copy. In most circumstances you only want one copy of it sitting around at any given time. It has developed to become sturdy and resilient to damage, and it is always under constant repair and error correction. It is also very long, and has the ability to be spun up into condensed packages for deep storage when not in use.

RNA is basically just a photocopy of DNA. Stuff all around the cell needs to use the DNA as instructions to do their tasks, but not everything can be swarming around the DNA reading it all at once. Instead, special proteins periodically "scan" the DNA and "photocopy" it to RNA. RNA is built similarly to DNA, but it is very short, and its structure makes it much more temporary. It lasts just long enough to leave the place where the DNA is stored, make it out to something that will read its bite-sized instruction, and then it disintegrates back into pieces that can be recycled to make new strands of RNA.

You can think of it like having one master copy of a very fancy and expensive book, that everyone in a company needs to read from from time to time. But instead of letting everyone mass around the book every time they need something, you have some employees occasionally flip to certain pages and photocopy them, and they send out photocopies to everyone. These photocopies are read a few times, thrown away, and then the paper is recycled.

Thermo - heat

dynam - change

ics - of or relating to, the study of

Thermodynamics. Of or relating to how heat changes. Or slightly rephrased, the study of how energy flows from place to place.

Energy flow is what allows anything to do anything. Keyword being flow -- all the energy in the universe won't help you do anything if it has nowhere with relatively low energy to flow to. A waterwheel set up on a small rushing stream will turn, but the same waterwheel on a glassy smooth sea will not. Studying thermodynamics helps you discover where all the proverbial streams are and how to tell them apart from useless seas.

Thermodynamics' sister, physics, will tell you how things move and bounce around after being kicked off. Thermodynamics tells you what kicked them off in the first place.

Politics is essentially the study of disagreements, and ways to settle them. This usually refers to the governments of nations when unspecified, but it more broadly applies to everything from company management to organized crime to school boards to homeowners associations. Anywhere two or more people disagree on how something is done and how things should be run, and it becomes a struggle over the power to decide, it's politics.

It'd probably be more intuitive to call it "stellar spray".

"Wind" implies that there's already a medium hanging around, and "wind" is just what we call it when it starts to move around. That's not really what's going on. There is no medium quite like that in space (I mean, technically there is, but the absolutely miniscule amounts of it is not what stellar winds are made of).

What stars (like the sun) are doing is basically sneezing particles outward in all directions at all times. Like when you see a person sneeze in just the right light and you see the spray of droplets fly out into the air. There is a kind of flow to it, like wind currents, but it's not a flow of stuff that was already there, it's a cascade of stuff being ejected out from a source.

In my view, a Sankey is strongest when it also behaves like a flowchart, using the left-to right axis demonstrating a sequence of events rather than a hierarchy of categories. There's a comment chain in here complaining about the number of employment search Sankeys posted to this sub; as low-effort and overplayed as those posts may be, I think a Sankey suits that kind of use case optimally.

In this situation, where the data is purely hierarchical, I feel it would be clearer as a bar graph. Either a series of bar graphs broken up by manufacturer or a stacked bar graph. The Sankey here is doing... alright, I suppose. But the large amount of 1's in the data and how at the lowest level of breakdown the range of the data doesn't exceed 7, it's a little cluttered and difficult to parse.

IMO, I'd say the Sankey has the same weaknesses as pie charts.

Sankey diagrams are almost never the right answer.

It's called a Sankey diagram.