Two lamells: zinc made and copper made are sticked into lemon - the battery is ready.

Lemon battery is an example of the galvanic cell, where current is generated due to electrochemical reactions. Each cell is made by two half-cells. This half-cell is an metallic electrode immersed in suitable dilution. On electrodes occurs oxidation reactions (on anode, ie. negative electrode) and reduction (on cathode, ie. positive electrode). Half-cells are connected each other with wire allowing electron travel from negative electrode to positive one. In our case as electrolyte we have water solution of different acids which are contained in every vegetable or fruit. At zinc plate there is oxidation, co zinc is an anode:

Zn_(s) --> Zn²⁺ + 2e

Zinc ions travel from surface of the anode into the solution, and electrons travel drom zinc plate to copper plate. On copper plate occurs reduction, so it is a cathode. ELectrons which arrive through wire are collected on the cathode surface and from there are used in reaction:

Cu²⁺ + 2e --> Cu_(s)

In that way copper ions are deleted from solution, and in their place diffuze new ones. In a cell there spontaneousli occuring reaction create current.