The properties of the stuff itself — the elements and what they are used for, how a fluid behaves as it warms, and what actually sets the strength of a concrete mix. The numbers other calculations start from.
Twenty-six molecules you can turn in 3D — and the shapes are worked out from electron repulsion, not looked up. Water bends to 104.5°, methane is exactly 109.47°, and C60 is built from the golden ratio.
All 118 elements on real grid coordinates, a nucleus and electron shells you can spin, five uses and five compounds each, and the table coloured by any property you like.
Density, dynamic and kinematic viscosity for water, oils, air and more, across temperature — the numbers every pipe, bearing and heat calculation starts from.
Cement, sand and aggregate by grade, with the water-cement ratio that actually decides the strength — and how much the dry ingredients shrink when you mix them.