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Since a redefinition that took effect in 2019, all seven base units are defined by fixing the numerical values of physical constants — not by physical objects, and not by properties of materials. It
Two theories from Einstein, built on the observation that the speed of light is the same for everyone regardless of how they are moving — a fact that sounds harmless and forces enormous conclusions.
A planet orbiting a star other than the Sun. Thousands are now confirmed, and the striking thing is that almost none were detected by seeing them — planets are overwhelmingly outshone by their stars,
Because the definition of "planet" was written for the first time in 2006, and Pluto fails one of its three conditions — not because anything about Pluto changed, but because what we knew about its
A form of matter that does not interact with light — it neither emits, absorbs nor reflects it — but which has mass and therefore gravity. We infer it because several completely independent
Stars form when gravity overcomes pressure in a cloud of gas and dust, and they die when they run out of fuel to resist gravity with. The entire life of a star is a standoff between those two forces,
Privacy is keeping something to yourself that does not affect your partner; secrecy is concealing something that does. The test is not whether you told them but whether they would reasonably consider
Because compatibility is not a fixed property of two people — it is a relationship between who they each are at a particular time, and people change on independent schedules. Most relationships end
A network of people who take on the functions of family — support, obligation, celebration, care in crisis — without biological or legal relation. The term originated in LGBTQ+ communities, where
They are formed by proximity rather than choice, exist inside a structure of power and competition, and — most significantly — usually end when the job does, which is the fact most people discover
No — and separating the two is the most useful distinction in the subject, because conflating them keeps people in situations they should leave and prevents others from putting down something they
Usually more disruption than the secret's content justifies — because the harm is rarely the fact itself and almost always the discovery that you were deceived, and by whom. What families keep
They match users through a combination of stated preferences, behaviour and engagement optimisation — and the honest answer on incentives is that a subscription business has a structural interest in
The double empathy problem is the finding that difficulties in communication between autistic and non-autistic people are mutual — a two-way mismatch rather than a one-sided deficit. It reframes a
It has to be renegotiated from a hierarchy into something closer to a partnership between adults — and most of the difficulty comes from the fact that this renegotiation is rarely explicit, so both
Because they are the only close family relationship you acquire without choosing it and without a shared history — and because everyone involved is negotiating a boundary that has never been
Both split a large dataset into pieces. Partitioning divides a table within one database; sharding distributes data across separate database instances. The distinction matters because one is an
Normalisation organises data so that each fact is stored exactly once, eliminating redundancy and the update anomalies that come with it. Denormalisation deliberately reintroduces redundancy to make
Because OFFSET tells the database to fetch and discard rows — so it gets slower the deeper you go — and because the underlying data changes between requests, which makes results skip and duplicate
A protocol providing a persistent, bidirectional connection between client and server over a single TCP connection — so the server can send data without being asked, which ordinary HTTP
Kept out of source code entirely, injected at runtime, scoped narrowly, and rotatable without a deployment — and the reason this matters is that a secret committed to a repository is permanently
Delegated authorisation — letting an application act on a user's behalf against another service, without the user handing over their password. It is frequently described as a login mechanism, which
A JSON Web Token is a signed, self-contained token carrying claims — typically who the user is and when the token expires — which a server can verify without a database lookup. That property is its
Cross-site request forgery makes a user's browser send a request they did not intend, to a site where they are already authenticated — exploiting the fact that browsers attach cookies automatically
An attack where attacker-controlled content is executed as JavaScript in another user's browser, in the context of your site — which means it runs with that user's session, cookies and permissions.
An attack where user input is interpreted as SQL code rather than as data — and it happens for one reason: the query was built by concatenating strings, so the database cannot tell your instructions
A property of a language or system guaranteeing that a program cannot access memory it should not — no reading past the end of a buffer, no using freed memory, no dereferencing invalid pointers. Its
Managing change in an interface other people depend on — where the governing constraint is that once an API is published, you no longer control when consumers update, and in many cases they never
A good name states what the thing is, in the language of the problem, so a reader does not have to reconstruct it from the code. Naming matters because code is read far more often than it is written,
Because estimating software means estimating work that has never been done before — if it had, you would copy it — and human judgement about novel work is systematically biased in a direction that is
A genuine design disagreement rather than a settled question — and the honest answer is that the two approaches differ in what they make visible in the type system and what they make easy to ignore.
Log what you would need to diagnose a problem you cannot reproduce — and never log anything that turns your log store into a liability. Most logging is written for the moment it is added rather than
Three mechanisms for coordinating concurrent access to shared state, at different levels of abstraction — and choosing the wrong one is how concurrency bugs are introduced. Atomic operations. A
Which lines were executed while the tests ran — and nothing at all about whether those lines were checked, or whether the behaviour is correct. It is a measure of what was not tested, which is
Replacing a real dependency with a controlled stand-in, so a test can run fast, deterministically, and without touching a network, database or clock. It is essential and it is the most commonly
A practice of writing a failing test before the code that satisfies it, in a short cycle — red, green, refactor. The evidence for its claimed benefits is mixed and weaker than advocates suggest,
By addressing the hazards that actually cause serious injury, which differ substantially from the ones parents worry about — and by recognising that childproofing is age-specific, since the risks
Through handling real money with real consequences, early and repeatedly — because research on financial capability consistently finds that attitudes and habits form far earlier than financial
By brushing twice daily with fluoride toothpaste and controlling the frequency of sugar rather than the total amount — and yes, fluoride matters, with an evidence base among the strongest in
Constant, undistracted supervision within arm's reach for young children — and the reason it matters so much is that real drowning looks nothing like the dramatic version, which is why it happens
Largely by not talking about weight — the evidence is unusually consistent that conversations focused on a child's weight are associated with worse outcomes, including disordered eating, while
By treating the parental relationship as a continuing working arrangement rather than an ended personal one — because the evidence is consistent that what harms children is not separation itself but
Plainly, honestly, and using the actual words — because euphemisms create confusion and fear in children who interpret language far more literally than adults expect. Why euphemisms cause harm. "Gone
A consequence is connected to the behaviour and teaches something about it; a punishment is imposed to cause discomfort and is frequently unrelated. The distinction matters because the evidence on
There is no evidence-based correct age, and the more useful question is what the phone is for and what it will be able to do — because the device is not one decision but several, and they can be made
Because adolescence involves a mismatch in the timing of two developing systems — the reward-seeking system matures early, while the control system that moderates it matures years later. The gap
Because judgements about accents are judgements about the people who use them, transferred onto the sound — there is nothing in the phonetics of one accent that makes it more correct, clearer or more
Writing whose purpose is to help someone do something or understand something, judged entirely by whether they succeed — which makes it the one form of writing with an objective quality test. The
Independently, at least three times, in different parts of the world with no contact between them — and in the best-documented case it emerged not from a desire to record language but from
The right to publish a work in another language, sold separately from the original — and the structure surprises authors, because translation rights are licensed by language and territory, not sold