It only took one brain cell to tap the letter “i” on my keyboard to begin this essay. This cell—a neuron—is nestled just above my left ear in the outermost millimeters of my brain. Under a microscope, it looks like a pyramid with flowering branches growing in all directions. It sends out a fine cable from its base which courses through the globe of my brain, down my brainstem, and into my spinal cord. Near the base of my neck, this little fiber releases a chemical signal onto another neuron, triggering an electrical impulse which wires its way through my right arm until it instructs a muscle in my forearm to contract, bending my finger and depressing the letter “i.”