  Fracture limits the structural stability of macroscopic and microscopic
materials, from beams and bones to microtubules and nanotubes. Despite recent
progress, fracture control continues to present profound practical and
theoretical challenges. A famous longstanding problem posed by Feynman asks why
brittle elastic rods appear almost always to fragment into at least three
pieces when placed under large bending stresses. Feynman's observation raises
fundamental questions about the existence of protocols that can robustly induce
binary fracture in brittle materials. Using experiments, simulations and
analytical scaling arguments, we demonstrate controlled binary fracture of
brittle elastic rods for two distinct protocols based on twisting and
nonadiabatic quenching. Our experimental data for twist-controlled fracture
agree quantitatively with a theoretically predicted phase diagram. Furthermore,
we establish novel asymptotic scaling relations for quenched fracture. Due to
their general character, these results are expected to apply to torsional and
kinetic fracture processes in a wide range of systems.
