You would be forgiven for assuming that we understand the proton. It is, after all, the main constituent of matter in the observable universe, the fuel of stellar furnaces. Studies of the proton—its positive charge suitably bound up with a negatively charged electron to make a hydrogen atom—initiated the quantum-mechanical revolution a century ago. Today researchers trigger torrents of ultrahigh-energy proton collisions to conjure particle exotica such as the Higgs boson.