Guides
Longer write-ups on the questions behind the tools: where a formula comes from, how a format works, what a simulation is actually showing.
BMR and TDEE Formulas: A Century of Equations, How Accurate They Are, and Why the 3,500-Calorie Rule Misleads
Where the Harris-Benedict, Mifflin-St Jeor and lean-mass BMR equations came from, what a systematic review found about their accuracy, where activity multipliers fall short, and why a 500-calorie daily deficit does not keep producing a pound a week.
Chaos and Sensitive Dependence: From Lorenz's Rounded Number to the Butterfly Effect
How a number rounded to three decimals in 1961 exposed the limits of weather forecasting, how Lyapunov exponents set a forecast horizon, why deterministic is not the same as predictable, what fractal basin boundaries add, and what Lorenz's 1972 butterfly talk actually said.
Color Contrast and Accessibility: Where the WCAG Formula Comes From, Its Limits, and the Law
Why text contrast matters, how WCAG's 4.5:1 threshold was derived, what critics and APCA get right and wrong, what Korean, Japanese, EU and US rules actually require, and where real palettes fail.
The Double-Slit Experiment: From Thomas Young to Single Electrons, and What 'Observation' Really Means
What Young actually showed in 1801 and 1803, how Merli, Missiroli and Pozzi (1974) and Tonomura's Hitachi team (1989) built fringes one electron at a time, and why which-path information and decoherence, not a conscious observer, wipe the pattern out.
How Simple Rules Build Complex Worlds: From the Game of Life to Phantom Traffic Jams
Emergence through five classic results: Conway's Game of Life (1970), Langton's ant (1986), Reynolds' boids (1987), the Nagel–Schreckenberg traffic model (1992) and Sugiyama's jam-without-a-bottleneck experiment (2008). Why a few lines of rules can produce behaviour nobody can predict.
How Image Compression Works: JPEG, PNG, WebP and AVIF Explained
Lossless versus lossy, JPEG's DCT and quantization, 8×8 blocking, chroma subsampling, PNG filters and deflate, and where WebP and AVIF came from — plus when to use each.
How Password Storage Works: From Plaintext to Argon2
Services don't store your password as you typed it. Here's the thirty-year climb from plaintext to hashing to salting to slow KDFs — and what the 2012 LinkedIn breach taught everyone.
How PDF Files Work: Objects, the Cross-Reference Table, and Why PDFs Are Hard to Edit
Open a PDF in a text editor and you find numbered objects, a table of byte offsets and compressed drawing commands. Here is how that structure works, where it came from — Acrobat in 1993 to ISO 32000-2 — and why a PDF can't be edited like a Word file.
From Gross Salary to Take-Home Pay in South Korea (2026): Social Insurance, Withholding Tables and the 3.3% Freelancer Rule
How a Korean salary becomes take-home pay in 2026: National Pension at 9.5%, health insurance at 7.19%, long-term care and employment insurance, the pension earnings ceiling, the simplified withholding tax table, the tax-free meal allowance, 3.3% freelancer withholding and the KRW 10,320 minimum wage, checked against official Korean sources.
PDF/A and PDF/X Explained: Archive-Grade PDFs, Print-Ready PDFs, and Who Actually Requires Them
Why PDF/A demands embedded fonts, no encryption and color profiles; how parts 1 to 4 differ; what the U.S. National Archives, Korean public records rules and Spain's LexNET really ask for; why printers want PDF/X; and how to validate with veraPDF.
PDF Signatures Explained: Signature Images, Digital Signatures and What the Law Says
A pasted signature image and a certificate-based digital signature are technically very different things. How each works, what PAdES adds, and how the US ESIGN Act, EU eIDAS, Japan's Electronic Signatures Act and Korea's 2020 reform treat them.
What Your Photos Say About You: EXIF, XMP, IPTC and Location Privacy
What EXIF, XMP and IPTC metadata contain, real cases where GPS tags revealed a location, how iPhone, Galaxy and Google Photos let you share without location, and a checklist for before you share.
The Reynolds Number: From a Streak of Dye in 1883 to the Dimples on a Golf Ball
How Osborne Reynolds found the boundary between smooth and turbulent flow with a thread of dye, why scale models in wind tunnels struggle to match it, and why a dimpled golf ball flies farther than a smooth one.
Making Scanned PDFs Searchable: How OCR Works and Which Scan Settings Matter
How OCR reads a page (layout analysis, then line recognition), what Tesseract 4's LSTM changed, where the 300 DPI rule comes from, image PDFs versus searchable PDFs, and why CJK text and handwriting are still hard.
How Korean Severance Pay Works: Average Wage, DB vs DC Pensions, Interim Settlement and the 14-Day Deadline
A guide to South Korea's statutory severance pay (toejikgeum) based on the Employee Retirement Benefit Security Act and Labor Standards Act: who qualifies, average versus ordinary wage, DB and DC pension plans, interim settlement, payment deadlines and retirement income tax.
Unicode and UTF-8 Explained: From Mojibake to Emoji
How text went from 128 ASCII codes and incompatible national encodings to one Unicode code space, why a design sketched on a diner placemat in 1992 became the web's encoding, and the normalization and length bugs that still catch developers.
Why Some PDFs Compress 90% and Others Barely Budge
Where the bytes in a PDF actually go — images, fonts and content streams — how the Flate, DCT, JBIG2 and JPX filters work, what object streams and MRC do, and practical recipes for shrinking each kind of file.