Batteries are comprised of two different metals suspended in an acidic solution. With the Fruit-Power Battery, the two metals are zinc and copper. The zinc is in the galvanization on the nails, and the pennies are actually copper-plated zinc. The acid comes from the citric acid inside each lemon. The two metal components are electrodes, the parts of a battery where electrical current enters and leaves the battery. With a zinc and copper setup, the electron flow is out of the penny (copper) and into the nail (zinc) through the acidic juice inside the lemon. In the exchange of electrons between the zinc and the copper over the acid bridge, copper accepts two electrons from zinc which accounts for the current. Once the Fruit-Power Battery is connected to the LED, you’ve completed a circuit. As the electrical current passes through the LED, it powers the LED and then passes back through all of the lemons before getting to the LED again. By the way, an LED is polar sensitive. That means an LED will glow only if the current is flowing through it in the right direction. If you hook up the LED and it doesn’t glow, switch the alligator clips attached to its legs. That should do it.