A battery is a device that contains materials that, when connected, trigger a chemical reaction that yields an electrical current, which is simply a stream of electrons. The transfer of electrons from one metal to another can be induced by putting them in the presence of an electrolyte solution. Commercial batteries contain highly corrosive acids, but it's possible to build a homemade battery with safer and easier to obtain materials.
Prepare the anode. Aluminum loses electrons faster than copper, so it will be the source of our electric current, called an anode. To prepare the aluminum foil anode, cut 10 circles with the scissors to the exact size of a penny.
Cut the paper towel. The paper towel will hold the electrolyte solution, and act as the medium to bring it in contact with the cathode and anode. To prepare this, cut out 10 squares just large enough to circumscribe a penny.
Stack and tape. To make the battery requires stacking the pieces in the correct order. Begin with a penny, the cathode, and put a paper square on top of it. Then place an aluminum circle directly above that. This represents one simple battery cell. Continue stacking in the same order, with a penny followed by paper, foil then paper. When finished, there should be a penny at one end and aluminum foil on the other. Wrap once with electrical tape to hold together without completely sealing off the contents.
Dip into solution. The entire stack must be submerged in an electrolyte solution for it to soak into the paper towel squares. Salt water can be used, but lime juice, vinegar, diluted bleach or even cola will produce good results. Experiment with different stacks in different solutions if desired.
Connect wire to terminals. Using the electrical tape, connect one length of wire to the penny on one end and the other to the aluminum foil on the opposite side. These represent the positive and negative terminals of the battery. When the wires are connected to a conductive material to complete the circuit, a small electric charge is produced.
TL;DR (Too Long; Didn't Read)
Use a voltmeter to measure the strength of your homemade battery.
To increase the charge produced by the battery, increase the number of pennies and foil slips in the stack.
Once the paper towel is soaked and the wires are attached, the entire battery can be wrapped in electrical tape if desired, to improve appearance and reduce chance of a short circuit.
Even though the charge produced by your battery is small, it can still deliver an uncomfortable shock. Avoid touching both wires at the same time.