how about a citrus cell? This remarkable lemon-powered clock designed by television's Bill Nye, the Science Guy, is certain to spark some curiosity.
MATERIALS
Thin cardboardScissors
Aluminum foil
3 lemons
Metal paper clips 3 two-inch pieces of 1/2-inch-diameter copper tubing
3 feet of thin copper wire
Tape
Button-battery-powered LCD clock (such as an inexpensive stick-on car dashboard clock)
DIRECTIONS: Cut the cardboard into three 1- by 4-inch strips and wrap each in aluminum foil. With a knife (parents only), cut a slit into each lemon, insert an aluminum strip and affix a paper clip to the top, as shown. Cut another slit next to the first and insert the copper tubing. The two metal rods should be close together but not touching.
Wire your lemons into a circuit, as shown: Connect the aluminum in lemon A to the copper 
in lemon B and the aluminum in lemon B to the copper in lemon C. Then attach one wire to the copper in lemon A and leave the other end free. Attach the final wire to the aluminum in lemon C and leave the other end free.
Now, tape the free end of the wire from lemon A to the clock's positive terminal (see drawing) and the free end of the wire from lemon C to the negative terminal. Within a minute or so, the clock should start to run.
WHY IT WORKS: When the two metals react with the citric acid in the lemons, an electric current is created. Store batteries work the same way. A charge is created by putting two different metals in an electrolyte--a liquid (usually an acid) that can conduct electricity. When the reaction ends, the battery is dead.
OH, IT'S NOT WORKING? Check all your connections. Reinsert the aluminum strips so they are closer to the copper tubes (but again, not touching). Squeeze the lemons to make sure they're not dried up (juice should come out).
ANOTHER TRICK: Find a dime and penny dated before 1987 and wash them thoroughly. Have a parent cut two slits in another juicy lemon, about 1/4 inch apart, and insert the coins halfway into each slit. Stick out your tongue and touch both coins simultaneously. You'll feel it tingling--that's electricity!
