An answer to Wheeler’s question “How come the quantum?” is proposed. Our answer goes back to an approach by Dirac and proposes a topological origin for Planck’s quantum of action. We propose a new fundamental principle: the quantum of action h-bar is due to twist-induced switches of crossings among fluctuating strands of Planck radius embedded in three dimensions. Due to their small radius, strands are invisible, though physically real. Nevertheless, crossing switches due to Dirac’s trick couple to photons, are thus observable, and define all physical observables. We verify the consequences of this fundamental principleagainst the basic quantum effects. The principle yields the appearance of particles as rationaltangles of strands, their indistinguishability, spin, and statistics. The principle also yieldsthe quantization of action and of angular momentum, wave-particle duality, emergence ofwave functions and Hilbert spaces, interference, Feynman’s path integral formulation, theSchrödinger equation, Heisenberg’s indeterminacy relation, non-commutativity, entanglement,the properties of photons, probabilities in measurements, Born’s rule, decoherence,spinors, chirality, antiparticles, and the Dirac equation. Strands also yield the principle ofleast action. The topological origin of the quantum of action appears to agree with all observationsand to be unique. Strands go beyond quantum mechanics: they determine leptonmasses, solve the mass hierarchy problem, and are consistent with the properties of physicalspace, with quantum field theory, particle physics, and general relativity.