Lemons are acidic, which means they have H+ ions in them. H+ ions conduct electricity well (positively charged particles). If lemon juice is put in one container with a bar of Zn (zinc) which is connected to a metal wire (which you will then connect to the clock), then get another container with a bar of Cu (copper) in it which is connected to another metal wire (which you will connect to the other side of the clock) and fill that container with a base (bases have OH- ions in them) (negatively charged particles), then also put a "salt bridge"(can be a glass tube with 2 right angles(looks like the letter "n") filled with salt water without any air bubbles in it(you should put a salt "cork" in both ends of the tube to seal it so no salt water gets out)) between the containers (purpose of the salt bridge is to complete the circuit). This procedure is effectively like making a battery. This experiment makes use of electricity produced by the oxidation of the Zinc bar and the reduction of the Copper bar. It will supply power for a clock. Science fair projects address this.