How lessons are made

Every unit goes through the same steps before it is published.

1. Sources first. Facts, dates and numbers come from primary or institutional sources: government agencies (NASA, NOAA, USGS, NIH, USCIS), standards bodies (W3C, IUPAC, NIST), museums, universities and openly licensed textbooks such as OpenStax. Each course file lists its sources with the date they were checked.
2. Numbers are recomputed. Answers to calculation questions, values shown in 3D models and figures in tables are recomputed in code. If a source rounds a value, the lesson says "about".
3. Two languages, written separately. English and Korean versions are written for each audience rather than translated word for word.
4. Checked on screen. Every page is opened on a phone-size and a desktop-size screen, in light and dark mode, and checked for layout problems, broken widgets and console errors. 3D models are checked for actually drawing on screen.
5. Simplifications are labelled. When a model is not to scale or simplifies the real thing (electron shells, orbits, a heartbeat's timing), the caption says so.

Writing and checking are assisted by software tools, including AI models. Facts are not accepted from a tool's memory; they must come from a source listed with the course. If you find a mistake, please tell us on the contact page.