You will need a cup, a 9 volt battery, two paperclips, electrical tape, some wire, a light, and some salt. The only tools needed are a pair of wire cutters and some wire strippers.
Step 2: Put it all together
Bend the paperclips straight. Then put them along the sides of the cup so that they are touching the bottom, bend the excess over the edge of the cup. You may have to tape the paperclips to the cup so that they stay. Next, take a length of wire and strip off the ends. Attach one end to a paperclip and the other end to a battery terminal. Then take another length of wire and strip the ends off. Attach one end to the second paperclip and the other end to the light. I chose to use an LED, but it can be any small light. If you use an LED, make sure you get the polarity correct so it will work. Lastly, take another piece of wire, strip the ends, and run it from the light to the battery. Use tape to fasten all of the connections.
Step 3: Test It!
Sprinkle some salt in the bottom of the cup. This will improve the conductivity of the water. Then put a small amount of water in the cup, just enough to cover the bottom. That should complete the circuit and the light should turn on. If not, then go back and check the connections. Once it works, you can extend the wires and run the cup outside and have yourself a handy little rain detector.