Interactive notes on things that are easier to believe once you can move them. Every page states a quantitative claim and then lets you check it — the numbers on screen come from the closed form, not from something that merely looks about right.
The Classroom → — the lab as a course: units, objectives, and predict-then-check exercises over every lesson, each anchored to a claim with a named test. And when reading stops being enough: write a lesson yourself — one Rust function, one manifest, and the framework does the rest.
The rule for these pages. Physics demos drift toward looking right rather than being right, and the two are easy to confuse on a screen. So each page here is built the other way round: the relationship is derived first, checked numerically against a closed form, and only then drawn. Where a shortcut would have been invisible, it is documented instead of taken.
Concretely, that has meant things like: scaling a lens's drawn thickness to 1/f because the lensmaker's equation says it must; keeping both axes of a diagram on one scale so a parabola is not quietly skewed; and solving bounce times in closed form rather than stepping a simulation that loses a tenth of a percent of energy per impact.