Clocks A Battery is an example of electrochemistry - using a chemical reaction to generate electricity. In most batteries there are two different types of metal that, because of their properties, are capable of producing an electric potential. In order for the electricity to flow in situations other than within metals it needs an electrolyte - a solution that contains charged particles called ions.
Many foods can provide the electrolyte needed to make a battery.
Citric fruits in particular are good at this because among the ions present are those that make the citric fruits acidic - and acids often help with the electrical conductivity. Depending on what show you see - you might see a lemon light up a light buld, or orange juice run a clock.