A capacitor is a device for storing a small electric charge. When two conductive plates are separated by a small insulator called a dielectric, they produce an electric field. The strength of this field is called the capacitance of the capacitor. The thinner the insulator and the wider and flatter the conductors, the higher the capacitance. Aluminum foil is conductive while cling wrap is insulating. Both are flat and thin, making them ideal materials for building a do-it-yourself capacitor.
Strip the insulation off the ends of two pieces of copper wire. About an inch or two should be enough.
Lay a sheet of cling wrap down on a table. Try to spread it out completely flat with no wrinkles in it.
Lay a sheet of aluminum foil in the middle of the cling wrap. The aluminum foil should be at least a centimeter or so shorter and narrower than the cling wrap. The longer and wider the aluminum strip, the more electricity the capacitor can store.
Attach one wire to the edge of the aluminum foil sheet by taping it down with some cellophane tape. Make sure the wire is actually in contact with the foil.
Lay another sheet of cling wrap exactly on top of the first sheet. Then, lay a second sheet of aluminum foil on top and attach a second wire directly above the first.
Carefully roll the whole pile into a cylinder with the cling wrap on the outside. Then, cover the whole thing in cellophane tape to hold it together.