HOW COME: We can feel through touch

In "I Sing the Body Electric," a story by Ray Bradbury, an "electric grandmother" arrives to take care of a family of motherless children. This "grandmother" was a robot, but human grandmothers - and children - are electric, too. In fact, every body is electric. Just as the current running through a lamp cord powers a light bulb, the body's own tiny currents power each and every cell, enabling them to pump blood, secrete hormones, move limbs, sense the environment and think.

In the copper wires of your home, electrons jump from atom to atom, creating a current. But our body's currents don't run along tiny wires, and the currents aren't made up of wandering electrons.

How it works: An ordinary atom is electrically neutral, because its negatively charged electrons are perfectly balanced by its positively charged protons. If atoms weren't neutral, everything around you, from your desk to your dog to the dandelions in your yard, would be electrically charged.

Ions are atoms (or molecules) that have become electrically charged. These charged-up atoms have gained or lost electrons, upsetting their carefully neutral balance. An atom with too many electrons has a negative charge; with too few electrons, the charge is positive.

Our bodies' cell currents are made up of ions. In a complicated process, cells separate ions by pumping them through holes in the cell membrane, called channels. Like a wooden toy that allows only triangles to fit through one opening, squares through another, the channels allow certain ions to enter or leave. That keeps the charges - eager to unite, since opposites attract - separated on either side of the thin cell membrane.

Take nerve cells, aka neurons. A resting nerve cell has a negative charge on the inside, since it's composed mainly of negatively charged protein molecules, which can't pass through the membrane. The nerve cell has a kind of "pump" that moves sodium and potassium ions - both positively charged - into and out of the neuron. For every two potassium ions allowed in, three sodium ions are ejected. So when a neuron is resting, there are fewer potassium ions on the inside than sodium ions on the outside. The result is an electrical voltage difference.

Nerve cells use electricity to transmit messages over the miles and miles of pathways running through the body, up the spine, and into the brain. That's how you sense that your bare feet have just stepped on sharp gravel, or that a cat's fur is silky smooth under your hand. As a resting nerve cell swings into action to send out a signal, sodium channels open. Since the neuron's interior has a net negative charge, positively charged sodium ions naturally flood in. As the inside of the neuron becomes more positive, potassium channels open, and repulsed potassium ions stream out of the cell. The result is a kind of electrical current, triggering the channels on neighboring neurons to open and close, too - sending a signal across the universe of your body.