Humans and most mammals can determine the spatial origin of sounds with remarkable acuity. To accomplish this small daily miracle, the brain has developed a circuit that's rapid enough to detect the tiny lag that occurs between the moment the auditory information reaches one of our ears, and the moment it reaches the other. The mastermind of this circuit is the "Calyx of Held." Scientists have revealed the role that a certain protein plays in initiating the growth of these giant synapses.