For anyone who has heard that the brain is made of neurons and wondered what one actually does. The numbers are typical, not universal, and the graph is not to scale: a real spike rises about a hundred millivolts, seven times the gap between rest and threshold, and one input on its own nudges the voltage by a fraction of a millivolt, so it takes tens of them arriving close together, not three. Some inputs push the voltage down; those are left out. The dip after the spike is real: for a moment the cell cannot fire again. Most long axons are wrapped in myelin and the spike hops between gaps rather than gliding; the drawing glides. The gap at the synapse is about twenty nanometres, and a minority of synapses skip the chemistry and pass current directly.