Cut a large lemon into three pieces, or small lemons in half, and put each piece in a little dish to contain any mess.
Cut two small slits through the skin on the top of each lemon piece, a little way apart.
In each lemon piece, push a strip of zinc (or galvanized nail) into one slit and a strip of copper (or copper-coated coin) into the other slit. These metal strips are called electrodes. The strips should be both pointing upwards but not touching each other, either in the air or in the lemon.
Attach a wire to the top of each zinc strip, and link to the copper strip in another lemon piece, forming a linear chain.
Use clothes pegs or paperclips to add the LED bulb into the circuit (and so closing the chain into a circle).
If the bulb does not light, move the strips around in the lemon a little, squish the lemons to release more juice.
The bulb will fade as the juice chemicals around each strip are used up. Move the strips to a new spot to start the electricity flow again.
Add more lemon pieces and strips into the circuit to make a higher voltage battery, which will make more current.
One lemon piece and electrodes produces 0.7 - 0.9V, so by adding more lemons and metal strips into the chain, enough voltage is generated to power the LED.
How does the lemon battery work?
Electricity is a flow of small particles, called electrons (which are part of an atom). When electrons flow through a bulb, they make it light.
The zinc pushed into the lemon starts a chemical reaction with the acid in the lemon which releases electrons. Electrons flow from the zinc strip through the wire then out of the attached copper metal strip (which also undergoes a chemical reaction with the H atoms of the lemon acid).
This current of electrons, generated by chemical reactions between the metals and the lemon acid molecules, lights the bulb.
Apparently you can feel the (safe, low current) electricity flowing in this circuit: place a finger tip between the copper and zinc strips at the end of the chain, instead of the bulb.
I have seen circuits where all the strips are pushed into one lemon, but it is easier to see the circuit when lemon parts are separated.
Try with other juicy vegetables/fruit e.g. potato (potato has phosphoric acid in it, which apparently works well).