The penny and aluminium foil battery is one of the best household batteries you can make at home. Because it is much quicker and easier to make a stack of twenty pennies than, say, a link twenty lemon slices together with electrodes, it is much easier to build up a strong voltage with this battery than with a vegetable one. The trade off is that it can be finicky, and does not usually last very long. Even so, it is easy to build and well worth the effort.
Dissolve salt into water. Mix salt in room temperature or warmer water until no more will dissolve.
Cut out 20 paper pieces that are slightly bigger than a penny. You can use cardboard, plain paper or paper towel. The only thing that matters is that they are a little bigger than the pennies you are using.
Cut 20 pieces of aluminium foil out, measuring each to about the size of a penny. Don't worry about making them exactly the same size.
Make your first battery cell. Pick up a piece of paper with the tweezers and soak it underwater for several seconds. Then carefully place it on top of a penny so that it covers the whole surface. Finally, put a piece of aluminium foil on top of the paper, using the tweezers to centre it.
Make several more stacks on top of each other. Stack in the order: penny, paper, foil, penny, paper, foil and so forth. Keep going until you have gone through 20 pennies. You should have a penny on the very bottom and a piece of foil on the very top.
Secure your battery. Carefully wrap it up with cling film to hold it all together. You can get by without doing this step, but it is helpful, since it stops the battery from falling apart. Do not wrap the ends of the battery, since those are your terminals.
Take a Christmas light, electric buzzer, small motor or other little electric gizmo and attach it to the battery. Connect one terminal to the penny on the bottom and the other to the piece of foil on the top. It should shine, buzz or whirr, depending on what you are using.
The straighter you make the stack, the better. The more pennies you add, the more voltage your battery will produce. You can use nickels instead of pieces of aluminium foil if you prefer.
Corroded pennies will not work as well as shiny ones.