(also called Electromagnetic Cascade Shower) Electrons can create photons by interacting with a medium. In a similar way, photons can create electrons and their antiparticles, positrons, by interacting with a medium. So, imagine a very high-energy electron, impinging on some material. The electron can set photons into motion and these photons can, in turn, set electrons and positrons into motion, and this process can continue to repeat. One high-energy electron can set thousands of particles into motion. Albert Einstein's famous relation governing the equivalence of matter and energy ( E = mc²) governs this process -- namely, matter (electrons and positrons) can be creased from pure energy and vice versa. The particle creation process only stops when the energy runs out.