Pin the tail on the hydrogens: X-ray diffraction has been the method of choice for determining the positions of atoms within a crystal. However, the technique works better for atoms with higher atomic numbers and requires single crystals of a minimum size. Palatinus et al. used electron diffraction, a technique of increasing importance for analyzing very small crystals, to identify the positions of hydrogen atoms in organic and inorganic materials (see the Perspective by McCusker). Science , this issue p. 166 ; see also p. 136