With the aid of some sea slugs, chemists have discovered that one of the smallest conceivable tweaks to a biomolecule can elicit one of the grandest conceivable consequences: directing the activation of neurons. The team has shown that the orientation of a single amino acid — in this case, one of dozens found in the neuropeptide of a sea slug — can dictate the likelihood that the peptide activates one neuron receptor versus another. Because different types of receptors are responsible for different neuronal activities, the finding points to another means by which a brain or nervous system can regulate the labyrinthine, life-sustaining communication among its cells.