**[Measurement of the axial vector form factor from antineutrino–proton scattering](https://www.nature.com/articles/s41586-022-05478-3)** **An antineutrino beam measured the axial charge radius of the proton and it was found to be 0.73 femtometers.** Scientists study the structure of particles called nucleons by firing high energy particles at them. They use either charged particles or weakly interacting particles called neutrinos. By measuring how the particles scatter, they can learn about the distribution of electric charges and other properties of the nucleons. They have now used weakly interacting antineutrinos to measure the nucleon structure more accurately, without needing to make assumptions about the structure of other particles around it. This is a big step forward in understanding the structure of nucleons in the weak sector, and will help future studies of neutrino interactions.