Insert a galvanized (zinc covered) nail into one end of a lemon, then insert a short piece of copper wire into the other end. This creates a battery. As with all batteries, chemical reactions occurring within the battery generate a flow of electrons -- which is what electricity is. The lemon battery is not very powerful. Hook this simple battery up to a voltmeter, a device that measures the voltage of a circuit, and it will register less than one volt.
Take two lemons and insert a galvanized nail and a piece of copper into each. The galvanized nail is the anode of each battery, and the copper is the cathode. The anode produces electrons while the cathode accepts them. Hook the anode of one of your lemon batteries up to the cathode of the other. Now hook the two remaining electrodes up to a voltmeter. You will notice that the voltmeter registers a higher voltage. It should be close to twice the voltage registered with only a single lemon battery. Batteries wired in series add their voltages.
All material is made up of atoms, and all atoms possess electrons; however, electricity does not flow through all materials in the same way. Some materials, like silver, copper and gold, are extremely good conductors. Some materials are very poor conductors of electricity. To illustrate the variable resistance of different materials to electric flow, take several batteries wired in series and connect them via copper wiring to a voltmeter. Then replace the copper wire with steel wire. The voltmeter will register a lower reading. Although the chemical reactions producing the flow of electrons has not changed, the electrons flowing through copper encounter less resistance than those flowing through steel.
Set up a simple electrical circuit with a voltmeter connected to a battery through a measured length of copper wire. Now replace a section of the wire in the circuit with another wire of the same material and thickness, but twice as long. Try it again with a wire four times as long. As the wire gets longer, the reading on the voltmeter should decrease, reflecting increased resistance as the electricity travels through a longer distance.
Now take the same simple circuit, but instead of replacing a section of wire with longer pieces, use wires of the same length but increasing thickness. You should notice the opposite of the previous demonstration. Resistance decreases as the thickness of the wire increases, and the reading on the voltmeter should also increase. Electricity encounters less resistance going through a thick wire than a thin one.