According to About.com, the acidic juice of citrus fruits can generate electricity. In fact, all you need to turn on a regular household light bulb is wire, copper and zinc nails and a piece of citrus fruit.
The "fruit battery" experiment involves copper and zinc nails as electrodes and the citric acid in the citrus fruit as the electrolyte. This is what is known as a "wet cell." Alessandro Volta used this physics concept to invent the "voltaic pile," the first wet cell battery to generate electricity. The voltaic pile consisted of alternating copper and zinc metal disks with cardboard pieces soaked in salt solution between them.