Electronics whizzes will easily understand the lemon battery, but the rest of us might trouble with the physics though the visual is unforgettable! Save some of those whitish nails from your garage; "paper fasteners" can be bought at Staples (screenwriters use them to hold their scripts together). These form the zinc and copper electrodes; the acidic lemon juice is the electrolyte (salt water works as well). With some plain old steel wool from Home Depot and a little Kleenex, you're good to go! Read more here.