Magnetic reconnection converts stored magnetic energy into kinetic and thermal energy by breaking field-line topology. The Sweet-Parker model treats a thin current sheet of length L and width δ: inflow carries flux at v_in, outflow jets emerge at the Alfvén speed v_A. Mass conservation gives δ/L = v_in/v_A = S^(−1/2), where S = Lv_A/η is the Lundquist number. Solar flares require S ~ 10¹², making Sweet-Parker far too slow — motivating Petschek's faster model and collisionless reconnection.