The foundations underneath the rest of the site — trigonometry, a full geometry suite for 2D and 3D shapes, and statistics & probability, including a genuinely random physical simulation of where the normal distribution actually comes from.
Solve any right triangle from two known values, watch the unit circle trace sine and cosine live, and see the Pythagorean identity hold at every angle.
Area and perimeter for eight flat shapes, volume and surface area for eight solids — circles to torus, squares to spheres — each with a live, labelled diagram.
Standard deviation, z-scores and normal-curve probability — plus a real Galton board where balls fall through pegs at genuine random and build the bell curve live, differently every run.