The Gamma-ray Burst Monitor (GBM) on Fermi was designed to observe the sky in gamma-rays, and especially for observing gamma-ray bursts. GBM is also advancing our understanding of Terrestrial Gamma-ray Flashes (TGFs). TGFs are brief flashes of high-energy radiation emitted by thunderstorms, typically observed as gamma-rays when the detector is above the source. TGFs also emit charged particles which are constrained to follow the magnetic field, creating Terrestrial Electron Beams (TEBs). GBM discovered TEBs to contain an unexpectedly large positron component, showing that thunderstorms produce antimatter. Thunderstorms are powerful particle accelerators. Our most recent result shows that 1 out of 2600 lightning flashes is associated with a TGF -- TGFs are not rare.