Will, S.J., W.J., Brenda. Every member touches every other directly. No platform in the middle.
The minimum 3D solid. Maximally rigid. Every face a triangle. The cage that doesn't bend.
Four pure states forming a regular tetrahedron in the Bloch sphere. The optimal symmetric measurement basis. d²=4 outcomes for d=2.
Calcium phosphate Ca₉(PO₄)₆ — phosphorus-31 nuclei in tetrahedral arrangement. Matthew Fisher's quantum-cognition substrate. The phosphorus we're for.