How an XSS attack steals your login cookie A comment box can be used to steal your login. XSS means cross-site scripting: someone posts a comment that is really a script. If the site puts it into the page as is, every…
How an SSD stores data with trapped electrons Every file on your SSD is a pattern of trapped electrons. Each flash cell has a tiny insulated trap. A high voltage pushes electrons in, and they stay there with the power…
How an ORM works: from an object to SQL and back You write objects. The ORM writes the SQL. Ask for user 2 and it sends a SELECT. The row that comes back gets poured into a User object. Change the name and it sends an…
How an LRU cache decides what to throw out A cache only has so many slots. So what goes when it is full? Every item you use moves to the front. When a new one arrives and there is no room, the one at the back goes: the…
How a dynamic array grows when it runs out of room Your list just ran out of room. Here's what it does. An array keeps items in a fixed block of slots. When the block is full, it builds a new one twice as big and copies…
How a CSRF attack works, and how SameSite cookies stop it A page you never trusted can make your browser talk to your bank. Browsers attach your bank's cookie to every request sent to the bank, even one another site…
How a SQL cross join works: 4 × 3 makes 12 rows 4 colours and 3 sizes give 12 rows. A cross join pairs every row of one table with every row of the other. Nothing is matched, so the count multiplies: two tables of 40…
How a cron job works: five fields checked every minute Cron doesn't plan ahead. It checks the clock every minute. A cron line is five fields and a command. 30 9 * * 1-5 means 9:30 on weekdays. At 9:29 the minute doesn't…
How a CMS works: content and design, merged into pages One template makes every page on the site. A CMS keeps your posts in a database and the design in a template with empty slots. To build a page it drops one post…
How a circuit breaker protects a failing service Three failures in a row and the breaker trips. While it is open, calls fail at once instead of piling onto a service that is already down. After a pause it lets one test…
How a CI pipeline stops broken code before it reaches main One failing test, and the change never reaches main. Every push is built and run through the tests on its own. Here 9.99 plus tax rounds down to 10.78 instead…
How 5G works: shorter waves, wider lanes, one aimed beam 5G's fastest band uses radio waves about 1 cm long. Old 700 MHz phone waves are 43 cm. Up there a single channel can be 400 MHz wide, twenty times the 20 MHz most…
Why your map drifts in a tunnel In a tunnel your phone can't hear the satellites, so it guesses. It takes its last known spot, your speed and your direction, and works out where you should be now. Then it does it again…
The minute with 61 seconds: how leap seconds work Some minutes have 61 seconds. The clock reads 23:59:60. Atomic clocks tick perfectly, but Earth's spin wobbles. In 1972 the average day ran 3.13 thousandths of a second…
How a mouse works: 1,000 tiny reports a second Your mouse sends the computer a tiny report every millisecond. The sensor counts how far it moved, 800 counts an inch on this one. Each report is three bytes: the buttons…
How a keyboard knows which key you pressed A keyboard doesn't have a wire for every key. Keys sit where two wires cross. On an IBM Model M, 101 keys share a grid of 8 rows and 16 columns: just 24 wires. The chip powers…
How AI like ChatGPT picks the next word AI writes one word at a time. A language model gives every word it knows a chance of coming next. From "The sky is", GPT-2 gives "the" 15.2% and "blue" 8.6%. It adds a likely…
How an IP address works 192.168.1.42 is 32 switches. Each of the four numbers is eight bits, on or off. On a typical home network the first three numbers name the network, like a street, and the last one names your…
How an inner join works in SQL An inner join keeps only the rows that match on both sides. Four people, four orders. Three orders name a person who exists, so those pairs lock together. Ben has no order and order 103…
How an HTTP request works: a request, a status, a reply Every web page starts as a 62-byte note. Your browser sends GET /hello and the site's name. The server answers with a status first (200 means OK), then the page…
How an API works: a menu, a request, a known answer An API is a menu your app orders from. It picks one request, fills in the blanks (here: latitude, longitude, temperature), and gets data back in the same shape every…
How a webhook works: stop polling, get called Polling sent 286 requests. A webhook sent one. Asking every 10 seconds whether a payment cleared, 285 answers said nothing new, and the app still found out 6 seconds late…
How a virtual machine works (and what a cloud server is) Your cloud server is a slice of someone else's computer. A hypervisor takes one physical server, here 64 vCPUs and 256 GiB of memory, and slices it into pieces…
How a trie stores words, and finds every word that starts with ca Words that start alike share one path. A trie stores words one letter at a time. Car, cat, cart and care all walk over the same c and a, so eight words…
How a router decides where your data goes A router asks one question of every packet: which of my routes is the most specific match? It keeps a table of routes, each covering a range of addresses. A packet for…
How a rocket booster lands itself, in 30 seconds A Falcon 9 booster can't hover. Even at its lowest setting, its engine pushes twice the empty booster's weight. So it falls unlit, lights the engine at the last moment…
How a linked list works A linked list never moves its items. Each node holds a value and the address of the next node, so the nodes can sit anywhere in memory. To put 25 after the first node, an array shifts the five…
How a heap keeps the smallest number on top The smallest number is always on top. In a heap every parent is smaller than its children. A new number joins at the bottom and swaps up. A pop takes the top, moves the last…
How a GPU works: thousands of tiny cores, one step at once A GPU wins by doing the same step everywhere at once. A 16-core CPU would give each core 129,600 pixels of a full HD frame. An RTX 4090 has 16,384 small cores…
How a foreign key works: why the database refuses customer 9 Order 104 points at customer 9. There is no customer 9, so the database refuses it. A foreign key is a column that has to match a real row in another table…
How a firewall decides what gets in A firewall is a list, read from the top. Every packet carries a sender and a port. The firewall checks it against rule 1, then rule 2, then rule 3, and the first rule that matches…
How a quantum computer finds the answer, in two qubits Quantum computers don't try every answer at once. They cancel the wrong ones. Two qubits give four answers a weight each. One gate flips the hidden answer below…
How a primary key works: why a database refuses a duplicate id Id 3 is taken, so the new row bounces. A primary key is the column that names each row. The database refuses a row that reuses a key or leaves it empty…
How a port works: one address, 65,536 doors Your computer has 65,536 doors. The IP address gets data to the right computer. The port number gets it to the right app: web traffic knocks on 443, mail on 25, remote logins…
How a load balancer spreads requests across servers A load balancer never sends work to a broken server. It hands requests out in turn, skips a server that is stuck on slow jobs, and drops any server that fails its…
How a lithium-ion battery works: ions inside, electrons outside A battery works because electrons can't take the short way. Charged, lithium sits between sheets of graphite. In use, each one splits into an ion and an…
How a chip gets made: printed with light, layer on layer Every chip starts as a plain disc of silicon. It gets a coat that reacts to light. Light through a mask prints the pattern, the printed part is cut into the…
How a browser draws a page: tree, style, layout, paint Four steps turn code into the page you see. The browser reads the HTML into a tree of tags. CSS rules give each one its look, and anything set to display none is…
How a card payment works, from reader to your bank and back Your bank says yes in under two seconds. The money moves a day or two later. The reader sends your card and the price to the shop's bank. The card network…
How a canary release tests new code on a few users first A canary release lets a bad update hurt a few requests instead of all of them. The new version gets one request in ten. Its error rate is compared with the old…
How a JavaScript bundler turns many files into one Six files in. One file out. A bundler starts at your entry file and follows every import to the next file. A file two others import is read once. Then it writes them…
Why your phone's battery percentage is a guess Your battery percentage is a count, not a measurement. The phone can't see inside its battery, so a gauge chip counts every bit of charge in and out. Tiny errors pile up…
Why old light bulbs are really heaters An old light bulb is a heater that happens to glow. It heats a thin wire to about 2,700 K, and a glow that hot is mostly invisible infrared. Only about 5% of it is light you can…
Why computers count in 0s and 1s (it's about noise) Computers count in twos because two levels survive noise and ten don't. A wire carries a voltage, and every voltage wobbles a little. Squeeze ten levels onto one wire…
What your phone's signal bars mean: each bar is 10 times weaker One bar isn't a quarter of four bars. It's a thousand times weaker. The bars show how much of the mast's signal reaches your phone. On Android the steps…
Does dark mode save battery? Only at full brightness Dark mode saves real battery only when your screen is bright. On an OLED screen each pixel makes its own light, and a black pixel is simply off. Purdue researchers…
Why this Sunday has 25 hours (and your phone shows 1:30 twice) This Sunday lasts 25 hours, and your phone shows 1:30 twice. Inside, your phone counts time as one number: seconds since 1970. It never jumps. Clock time is…
Why real random looks rigged: streaks are normal Flip a coin 100 times and you will almost certainly get six in a row. People faking random switch sides too often, so real streaks look suspicious. The chance of a run of…
How your phone texts a satellite with zero bars Zero bars and your text still goes. It goes to a satellite 1,414 km up. Your screen shows which way to turn so the phone points at it, the message is squeezed to about a…
How your phone knows which way is up Your phone turns the screen because a speck of silicon sags. Inside is a motion sensor: a tiny mass on springs, with comb teeth between fixed teeth. Gravity pulls it down. Tip the…