Sitting in a darkened lab at the National Institutes of Health in 1999, my ­colleague Beth Stevens and I prepared to send a mild electric current through fetal mouse neurons in a cell culture. We were using a new microscope technique that would let us see ­electrical activity as a bright fluorescence emitted from a dye we had added to the culture, and we were hoping to find out if another kind of cell common in the nervous system would react in some way—Schwann cells, odd-looking cells that fabricate insulation around neurons. We didn’t really expect them to; Schwann cells cannot communicate ­electrically. I flipped the switch. The neurons immediately glowed. But then the Schwann cells began to glow as well. It was as if they were talking back.