Batteries are composed of two different metals suspended in an acidic solution. In the Fruit-Power Battery, the two metals used are zinc and copper. The acid comes from the citric acid inside a fruit, such as an apple or 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 electrons flow out of the copper strip, through the acidic juice inside the fruit and into the zinc strip. Once the Fruit-Power Battery is connected to the LED or clock, you’ve completed a circuit. As the electrical current passes through the LED or clock, it powers them.